I think that PLOX will have to be prepared to play Banette. However, think to yourself:
"Hm... Banette has a positive matchup against PLOX... but nothing else. Empoleon will own it, same with many other decks. Hm... plus, look at my shiny new Empoleons... or those Gardevoirs and Gallades I worked so hard to get... hm... I didn't invest too much for the Banettes... so, I'll play Empoleon or PLOX!"
See, people know that now that MD is released, many people will be playing cards from MD. It's a new set, and it has good cards in it (Eevees, Empoleon, Garchomp, etc.). People will play MD. People may still play PLOX, just because it's the BDIF and everything, but no one wants to lose to Majestic Dawn cards, wich will be played just as much as PLOX.
Remember, your opponent's aren't just people trying to beat you, they want to win the tournament too. So, they won't play a deck just to counter yours, they'll play a deck they think has at least a 50-50 matchup against almost all of the big decks right now.