Well how they would combine PC and Console on a server and make it fair was the issue. I know they were experimenting with it, but I have no info on actual results that have been claimed. I assume since it never happened it never worked. Too unbalanced towards the PC players.
I used to play PC games competitively (never really won anything) and am pretty good at FPS. After six months of MW2 on Live, when I got used to the difference in the controllers, I reached a rank of #53 on the MW2 Live rankings. Most people were in the hundreds of thousands ranking. I played it alot, yes, but I was pretty darn good at it. There were still lots of better players, and always will be, so I'm not bragging, just trying to show that I'm pretty good and experienced with these kind of games. I can absolutely say that mouse/keyboard is a far superior setup to a console controller. It's just not possible to play the same on a gamepad as it is with a mouse. In a negative way. I still have fun playing console games, but in the end the mouse/keyboard is just much better.
The reason some of the big leagues use consoles is because it has much more mass appeal and sponsorship opportunities than PC. Microsoft and Sony can offer alot more in terms of sponsorship and prizes than ATI or Nvidia. And early on they all competed on PCs and I remember they were even airing comps on TV, but the market was just too narrow. There aren't that many PC gamers out there compared to consoles, and I imagine the console gamers had little interest in watching PC comps. Going to console games like Madden and Halo opened up a much larger market.