Quote Originally Posted by thecrav View Post
In the situation he's provided, the guy has already looked at the deck. The problem is not just the guy being able to manipulate the deck but also the guy seeing what you're playing. For Julian (elves), seeing any one of ~50 of his cards will tell you a ton about the deck.
Keeping your hand or taking a mulligan is one of the hardest decissions in a magic game, and knowing what your opponent plays makes that decission much more easy. As an example, sometimes I've kept a hand expecting my opponent to play the same deck he played in the last tournament, and in the times that he's playing a different deck, I'm usually f***ed..