You shouldn't re-use addresses.
Satoshi made one post about the re-use of bitcoin addresses and everyone takes it to the extreme.
Please explain why someone should not re-use an address.
I'll save you the effort, because there are no valid technical reasons. There are "privacy reasons" (bearing in mind that bitcoin was never intended to be, or portrayed as, an anonymous payment method) why re-use may be considered less than ideal, but I'm tired of seeing people say you should not do it like it's a plague or something.
The only good reason that people shouldn't reuse addresses is to maintain privacy. By not reusing addresses, it becomes more difficult to monitor and track someone's payments. As a service provider, it would be easier to give each customer their own address, BUT, many bitcoiners do not like to have their payments to be easily tracked, and by generating new addresses for each deposit, a service will appease the privacy minded and could gain more business.
It is a bit odd. Bitcoin Forum has a field for "Bitcoin Address". I don't actually have a "Bitcoin Address", do I?
The Bitcoin Address field is a place for you to advertise your bitcoin address.
Makes me wonder. In general, if I want anonymity, there is no way for me to advertise an address of mine. I know that sounds obvious, but if I wanted to receive payments, someone would have to goto my site and ask me for one.
It is still possible to maintain anonymity even with advertising an address. You could send the Bitcoin received by that address to a mixer or use any other method of mixing and anonymizing Bitcoin. It will prevent others from knowing what your other addresses are.