IIRC, a plant produces most, if not all of the sugars (carbs) it needs through photosynthesis.
http://en.wikipedia.org/wiki/Photosynthesis
"
Photosynthesis changes sunlight into chemical energy, splits water to liberate O[SUB]2[/SUB], and fixes CO[SUB]2[/SUB] into sugar."
As MYOB mentioned, the main, proven, benefit of adding a sugar to the root zone would be to feed beneficial bacteria.
That is also where a debate between many growers arises:
"Can the end taste of the bud be affected by whatever is in the root zone?"
Some people say sugars and anything else will definitely not affect the taste of your bud. Other people say they can taste a difference between things like hydro vs. soil (when the only difference is the medium), flushed vs. not flushed, added sugars vs. not. And then even other people are half and half, saying some things affect taste and some things do not.
Whatever the case, I think it's best to keep an open mind...to know the truth, you really need to test for yourself. Do something like a SOG and blind taste tests. I'm sure it will be very easy to find people to try two different kinds of dank bud and tell you if they taste a difference. All the rest is internet hearsay anyways. And even if you do believe some thread or post on the internet about
any debated subject (defoliation, flushing, sugars, hydro v soil), you should still test for yourself just to make sure. That's the best part!