The most I know about using complex carbs is the baby oatmeal that Microbeman has stated helps grow more fungal hyphae. Since it's not a complete or whole grain, but is in fact so stripped down that it dissolves in water, I don't see it causing much harm as long as one doesn't use so much that it causes crusting on the soil surface.
As for benefit, I've gone back and forth, mentally, on that one. On the one hand we're told that mycorrhizae are generally slow-growing and better applied to perennials, whereas annuals prefer bacterial-dominated soils. But I know so many people who report differences when inoculating with mycorrhizae that it's impossible to ignore. So I've endeavored to keep live plant material in my beds at all times, so that any mycos will (hopefully?) remain in situ, growing well, etc.
