It makes sense that you don't trash old addresses in that you completely throw away your
private keys. But it makes a lot of sense to be able to transfer unused accounts somewhere else,
where you can still check up on them periodically but they don't clutter your wallet.
An 'Archived Addresses' tab would be nice to have. A typical non-techie user wouldn't want to, or necessarily know how to set up and transfer funds to a new wallet every now and then. It's also an unnecessary hassle. Sure it's not a huge deal to most of us current users, but if bitcoin is going to catch on with the masses, the client needs to be clean, feature-rich, intuitive, and it needs to make everything it does as simple and easy as possible.