Sorry to hear about your loss of BTC Steven, hope you manage to get something back!

I'd be interested to know what prevents online wallets like MtGox from:

• never giving an address used by one user to another user
• remembering all addresses used by a user and considering them all to belong to him

since that would seem to prevent these sort of problems.

Is it a technical reason - I thought addresses were cheap to
generate, but perhaps having to cope with lots of addresses
per user is tough?

Or is it more of a philosophical thing based on the original
intention that addresses aren't used over and over by the
same user (like how change goes to a different address)?