Pokémon Black and White versions

The point is to trick your opponent, though.

Player 1 sent in Tentacruel!
"oh okay, it's going to Rapid Spin, I'll send in Rotom"
Go, Rotom!
Tentacruel used Swords Dance!
Tentacruel used Crunch!

Oh pomegranate.

But it has to be sent out and transform in battle, from what we see. It doesn't transform before the match begins, it transforms during the match as soon as it's sent out, showing the other player that they didn't really send out the Pokemon that's shown. Of course, I do expect the change stays that way for the whole battle, but if you see the transformation once you should tread more cautiously whenever you see that Pokemon out. So if you see Zoroak transform into Tentacruel and switch out, you shouldn't try countering it with something that can get raped by Zoroak.

It just requires you to try reading your opponent a bit more for prediction. I can see it being a pain on some teams, but it's not that big of a surprise, because you know it's coming. You just have to try and predict correctly.