At live tables in cash games we are allowed to show one card before opponents make a final decision at most of the casinos. Why isn’t it allowed at the tournaments? And would you want to see a feature like this at the online tables?
No!
Because it is trying to influence a players decision, by showing one card before he decides, you could be faking that you hit the hand you totally missed like having a 4 card straight or the Nut flush with a bare Ace with nothing backing it up.
Sure after the hand is dead I would allow players to show their card's, one or even both, but why would you want to, giving information away freely is giving your opponents an advantage you don't have over them.
Keep them guessing as to whether you had the best hand or not is always the best strategy.