I used to. I had some unreliable players who would add extra equipment or attempt to raise their ability scores between sessions.
Also, the D&D groups in my area tended to have a lot of overlap. Often, players in my area would play in 2-3 groups and would take the same character to use in different games, gaining gear and XP in other adventures and then attempt to use the upgraded character in my game (extremely annoying when the characters in my game were in the middle of an adventure and someone would try this).
I also would not accept characters from other games into my own. If you wanted (and still do wish) to play in my campaign, the character had to start in the game world, grow, and stay there. (This was, of course, instituted after several new players would try to bring their magic-laden, higher-level-than-my-group PCs into the game).
I feel my current players are a lot more mature than my old group (though they are about the same age as my old group) and I've lost too many character sheets now that I generally prefer players to keep their own sheets when possible. I still don't allow other-campaign characters at my table, though.