I doubt anyone will seriously play that stadium just to counter two potential decks...
Well, sometimes you can use a stadium not only as a counter, but also to get rid of potential liabilities. Although this new Parallel stadium seems to give both players some kind of disadvantage, both sides can be useful depending on the state of the game: you can simply limit the bench size of your opponent and prevent him/her from using Shaymins, or from applying any fast deck strategies like Raichu, MRay or the new Break Zoroark, or you could simplify your side of the field getting rid of heavily damaged mon's on your bench, or low HP ones you played for support.
My main interest on this new stadium is that you can play it, like any other stadium, only if there is no other stadiums in play, or if there is a different one in there, but never if a stadium with the same name is active -- I mean, it would be a huge problem if the stadium is there locking you on the wrong condition, because your opponent plays the same stadium, for example. So, to get rid of the selected condition, you would have to:
1. Play some other stadiums on your deck, but then you would have to wait another turn to be able to play Parallel City again (you can only play one stadium per turn). Oh, and you would have to pray your opponent wouldn't play his Parallel City during his next turn, locking you from playing it again....
2. Play cards that discard stadiums, like paint roller. Which, by the way, seems to have some kind of synergy here against this stadium (even if you are locked out by Giratina EX or Ninetales, you still could get rid of the stadium and have some chance to play at least for a turn).
3. Use mon's that have Wreck or some other attack that discards stadiums. On the other hand, your opponent would have the advantage of not being locked by the stadium during his/her next turn.
4. Play that Teleport Room Gothitelle (FUF, if I'm not mistaken), but, then again, it is a stage two, and who even play that card anyways?
So, I'm curious if this new stadium would became a thing. I think its mechanic is very interesting and would bring a lot of cool strategies to the game
Also, I guess I'm going to miss backbiting seismitoad =( Locking is real, people! Now we are going to have items, stadiums, bench space, and special energies lock, all of them potentially starting turn 1 or 2, and everything could be put on the same deck =X