Since Trainer isn't only used for testing decks to be played against AI, it would be very helpful to have a PvP option to play against other players with decks that both players have created in Trainer. This way, you don't have to cross servers (if that's a problem), and you wouldn't put players on their account at any disadvantage against players who created decks using Trainer.
Its nice to have Trainer to create PvP decks and tweak them, even playing against AI. However, playing a deck against AI may give the illusion of success, when really the deck might merely exacerbate a "problem" with the AI, and exploit it to win. Playing against other players would test the effectiveness of the deck against real people. If we had the same options for PvP as on our accounts, ie PvP1 / PvP 2 / Duel, we would be able to use the same restrictions from the real game and the flexibility of Trainer to optimize PvP decks.