Well, I'm assuming you mean in Wi-Fi battles.
When you are entering a competitive online battle, there are hack censors that detect illegal Pokemon, and don't let them entera battle.
So really, PokeGenners aren't all that common. But they are still there.
Note, for your example of Transfer only moves, even though these are legal moves, the censors would detect that the Pokemon is from X/Y instead of BW(2). The censors aee generally pretty smart.
Hope I helped. :)