Consider this for a moment: the blockchain is publicly available to everyone, so every transaction done is publicly visible (that is how it can work decentralized).
So if you decide to use only one bitcoin address for all your transactions, once somebody knows your address (like the people that are paying you some coins), that somebody can see all your transactions. It's like making public your whole bank account !

For that reason, you should give different people different bitcoin addresses, unless you want your private transactions known to the whole world.

Of course, you are free to reuse any "old" address. The client keeps track of all your addresses (that's the wallet.dat file). Just beware of the consequences.

Thilo