As JWittz said, hopefully they will add some sort of ranking system in PvP, personally I think basic yellow should still be allowed, this is still a beta after all and as the game grows and people get a bit more experience they will start to get packs and build new decks, I don't think the basic yellow deck shouldn't be disallowed as it is somewhat unfair for the new guys even if they do choose expert.
For now the expert mode doesn't mean much, a win is a win, and a loss is a loss, there are no rankings or leaderboards, no rewards for winning or losing, sure it might get a bit boring after a while playing them, but just think of it as practice for you, getting better at playing, what decisions you should make, and sure enough eventually people will pick the correct mode suited for them, novice or expert.