[Cazz]

Its generally considered bad form because if you ask to see my hand, I have to show the whole table-- not just you. This gives the rest of the players free information on how to play against me.

In some sense, having just the bettor show is a way for you and me to give out the minimum amount of information. When the tables are reversed (I bet and you call), I will extend the same courtesy to you.

Since there is a pot to award, SOMEONE has to show first to determine the better hand. The logical person to show is the one that showed the most confidence in his hand by putting in the last bet or raise.

There are also a few practical points to not asking to see hands. You don't want to show up a calling station; let him keep calling. In some places, if you ask and the hand that is shown beats your hand, you will loose the pot.
Also, if someone asks to see my hand twice in a session, I will ask to see his hand on EVERY opportunity I get. In my card house I have to be in the hand at the river, but you can see this will start to give free info on your play to everyone.