I sent you .01 just now. Ended up installing 3.23 Bitcoin app, and now I have another address (already?).
The standard Bitcoin client likes to generate you a new address whenever you receive coins on one more than once. This is to encourage you to use a new address for each transaction, which is important if you want to be sure you know who exactly is sending what. Obviously this matters more if you're actually selling something to people and not simply playing Bitcoin Darts or whatnot. Without getting into an escrow system like ClearCoin, this is the best way to make sure that a specific person paid for whatever you're selling.
Other than cluttering up the address book, they do no harm, and addresses can never expire. I'd get into the habit of labeling each one though, just to be on the safe side.