[Lestat]

There are 4 reasons:

1. It slows down the game.

2. This is probably the most abused rule in poker. The original intent of this rule was put in place to prevent collusion. Unless you suspect collusion, it is poor etiquette to ask.

3. Howard Beale is correct when he says it can only be bad for the game. Poor players who know they're likely to be shown up, will all of a sudden start playing better.

4. One skill an expert has over others is his ability to read hands. When some donkey asks to see another player's hand, he is giving information to the whole table that previously only those who knew how to read hands were privvy to. This costs the better players and is why you almost never see one ask to see a hand.