Can it cause the motorcycle to move erratically or what?
It shouldn't do anything other than raise the engine's revs a bit, make a little noise out the muffler and go vroom!!! It's entirely unnecessary. If you have to do that to keep the engine running, you are out of tune. Fix the problem.
While braking for a stop you would normally apply the clutch too, so the engine is disconnected from the rear wheel, so throttling up would have little affect. If you were, for example, braking for a corner, released the brake and accelerated hard the back wheel may move sideways (it seems more when you are on the bike than it appears to an outside observer). In a straight line the sideways movement would be less (generally) but the front suspension will load up during braking and unload during acceleration giving a choppy movement (this will also happen in a corner and will exacerbate sideways movement). This is why braking while leaned is advised against.
If you were leaned over in a corner, it would wobble like poo, but in a straight line there shouldn't be any dramas
