Two things quickly to start:
First, as far as adding sugar to a DWC being a bad plan--I won't discount you there, it certainly does invite unwanted dance partners.
Second, as far as the chemistry goes--I apologize if it seems I'm trying to replace both hands on experience and all other scientific disciplines with chemistry. That is certainly not my intention. My only hope is for chemistry to be a part of the conversation--because it really is at the root (no pun intended). It is known as the central science for a reason :)
That said, I'll respond to a few of the things noted.
Because it was pointed out that chemistry is clunky and difficult to grasp without significant effort I'll include some links to youtube videos where I can which better explain the concepts. Additionally, I have--for some time--been compiling a chemistry primer and compendium aimed specifically at farmers. This guide (which is pending free time, something I'm a bit short on) will seek to leave nothing unturned and my hope is that it can become a liquid document which is added to by research from the community or any discussions which may come to light in a thread.
To start, we'll cover photosynthesis and the C3 pathway (glucose production).
http://www.youtube.com/watch?v=AUPugYBkNJQ&feature=related (Light Reactions)
http://www.youtube.com/watch?v=o5SRPeAHRpk (Dark Reactions)
After watching these videos it will be apparent that plants use a large amount of energy (both in the form of ATP and NADH+/NADPH+ reducing power). To produce glucose, and subsequently sucrose (glucose must also be phosphorylated to be joined with fructose to create sucrose).
Because of the nature of the proteins, thus far, which have been found to transport sucrose in roots (proton/potassium symports/antiports
http://www.youtube.com/watch?v=NewpaNwevFk ), it is easy to see that--energetically--it is cheap and beneficial to the plant for it to take in basically ANY sugar through this pathway. All sugars end up going through glycolysis when they are used for energy later. There are various shunts and offshoots to allow for maltose, fructose, and any number of other sugars to enter the pathway (these proteins are typically isomerases, or mutases--wiki them). Lactose intolerance stems from the lack of a starting protein in one such pathway (lactase, a hydrolase protein).
Here is a very detailed run through of glycolysis which I recommend--
http://www.youtube.com/watch?v=O5eMW4b29rg . For some insight, this video best describes and shows the way in which i really think of most of these processes. My training dictated that I memorize all these pathways, structures, enzymes, etc--and so it's hard for me not to want to use that info.
A second video discusses the Citric Acid Cycle:
http://www.youtube.com/watch?v=0mS0rxDxnAc
The important info I really want you to see from this video is:
A. this is where the energy production really occurs from glycolysis.
B. If you look at the end of the video of the entire circle of cycle with all the intermediates listed I want to point out to you (without having to post 5 more videos) that virtually the entire left half of the big circle represent intermediates involved in: nucleotide production, amino acid synthesis, waste removal, fatty acid synthesis, you-name-it. This really is the starting point for most everything.
Ultimately once you've completely metabolized a molecule of glucose you end up with 36ATP if you do all the math (NADH is later used to create ATP).
For one molecule of sucrose that number will be 71ATP ( you lose 1 ATP molecule in comparison with breakdown of glucose if using fructose as a starting point--this is debatable because when glucose-fructose are split, one of the molecules will be already phosphorylated, you can also end up with more under certain conditions ~76ATP).
72 ATP is a lot of energy for 1 molecule brought in by what amounts to fancy diffusion. Especially considering that virtually everything the plant does insists upon this molecule. It costs about 18ATP per sugar molecule produce in photosynthesis. Sucrose being a disaccharide requires two turns of this wheel for 1 molecule so about 36 ATP.
The proteins in the electron transport chain (ECPs) must also be constantly regenerated, and so must chlorophyll, and the iron-sulfur centers involved, These are VERY expensive if we're talking on an ATP scale cytochrome c being the smallest (I'm pretty sure) of the ECP's is 100ish amino acid residues. Corresponding to at least four times that many molecules of ATP (assuming the needed amino acid was free-floating in the cell already). Cyctochrome b is 4 times as large and is not the only protein involved. Chlorophyll's and carotenoids are similarly large and expensive molecules to make, and are also not the only other non-protein compounds needed. All that considered, remember that the plant specializes for this process and does not even have access to light more than half of the time during flowering--and even during that time experiences peaks and valleys in photosynthetic productivity based on local concentrations of sugars, availability of nutrients, and need for regeneration of molecules/proteins. That is to say, plants are severely invested in sugars. Comparatively--the protein responsible for sucrose uptake is probably fairly large (I'm not going to look it up lol), but it will not need regeneration as often as electron carriers and the only needed chemicals to power it are readily available ions.
Forgetting that last bit (as hard as it should be to), per photosynthetic cycle the net resultant energy is about 72-36 = 36.
The efficiency is essentially doubled through uptake, and this is forgetting the huge process the plant undertakes otherwise to get its hands on glucose. It does not make sense that an organism would evolve from a one celled organism interested in this molecule and then select out against symports/antiports which do, more efficiently, the same job the entire rest of the multicelled organism is aimed at doing.
It
makes sense that this process should happen--and that is probably why the research was done in the first place.
Now, that doesn't mean--and I never suggested it should--that putting a ton of sugar in your medium is going to save the day, or even be good for that matter. But from a biochemical, and even a biological point of view it stands to reason why a plant would do this.
If you want some examples of how this might be beneficial I'll conjecture for you:
1. More starting materials for protein synthesis, might aid in faster/more hearty root development.
2. As a result of 1 roots may respond better to stress. Often proteins/enzymes are the end product of a stress response, and they are intended to bring the cell back into a happy state. These can be produced, function, and degrade in under 1 minute numbering in the hundreds of thousands of proteins. A large amount of starting materials is absolutely essential for this process to go to its full potential.
3. More starting materials for waste removal--helps cell maintain concentration gradients, pH, and charge differential.
Those are just a few, and they would have an immediate impact on roots locally virtually as soon as uptake proceeded.
For a possibly bad thing (speaking only on the plant, of course this can invite unwelcome guests as has been stated), or good depending on how you look at it:
It's been shown feeding sucrose/glucose can suppress photosynthesis and cause leaf bleaching and breakdown of chlorophyll. This would be terrible early in the life of the plant--but could be good for those looking to make extracts where chlorophyll is an unwanted entity (and sugars are easy to remove).
For anyone wondering, yes leaves have also been shown capable of taking in sugars--but the same cautions apply with regard to microbial growth.
I never intended to be nasty, or even to claim to know-it-all. My favorite thing about myself is that I'm sure I don't know shit. I've always said--as was said to me by my grandfather--that science is mostly about making the best mistakes.
My only intention is to impart that same feeling about science to others. I think the LACK of that in the scientific community is exactly the reason why MJ has gone illegal for so long while somehow I can go buy a liter of Everclear down the block with the intention of drinking it and no one will try to stop me.
