I didn't get into Bitcoin early enough to be able to solo mine, but based on my experience solo mining with alts, I'd say it's by design that the client uses a new address to fund a block it has won.

I can't find any obvious quote at the moment, but I'm reasonably sure I've seen Satoshi say that address reuse is a bad thing. To support that, here's a post where he mentions a feature which automatically allocates a new address for an account once funds are sent to it: https://bitcointalk.org/index.php?topic=665.msg18349#msg18349