I know they are 'technically' illegal, but I don't think I'd do uscf race... do they work as well as canti or v-brake? my discs work SO GOOD on the mb... why not on a cross?

from what I understand, they work just as well as if they were on a mountain bike however it weighs down a bike a lot, since you want the least amount of weight as you can get on a cross bike then this would not be the way to go. If you're not using it for racing then this shouldn't be an issue since weight isn't as much of an issue.

why are disc brakes illegal?

why are disc brakes illegal?
Thank the UCI regs for that one. They ALWAYS take forever to approve new tech for their races and in the interim make the product illegal for use in said races until they eventually approve it. See Gary Fisher's pushing of the 29"er in the MTB world for a reference.

Only applies to UCI elite race catagories, like elite men, women and juniors. You can use 'em in masters, B and C races. But, I had heard through some people that they may extend the ban to master catagories, maybe next year.
The one problem is that you would have bring a second set of wheels or another bike w/ discs to a race because you wouldn't be able to bum a wheel from other racers. In NorCal, I think I may have seen one or two guys in the masters actually running discs; everyone else uses cantis'.

Why do you need additional stopping power in a cross race? You should be go, go, going! :D

I think you can use disks now?

Someone made a good point to me once that shouldering a bike with disks can be a bit dangerous.

If you hit yourself with the disc rotors, you're probably carrying the bike all wrong. I think the UCI said it was dangerous for other riders who may be closer to the wheel when running side by side. Even so, have you been hit by one of those FrogLeg brake arms? They dig in and hurt, they stick out a mile.