just one reason of many, i'd say.
Do share the other reasons as they come to you.
I don't see how what you've said so far is a problem.
Don't take me the wrong way, I am genuinely interested in where the existing functionality fails to suffice
okay well here's one more reason i can think of... let's say there are TWO types of ways a site can use bitcoin:
1) the kind of site where new temporary
addresses are constantly being generated for incoming payments, eg. btcflip.com
2) the kind of site where there is only ever 1 permanent
address generated per customer (I think tradehill might be starting to do this).
for a site in category 1, it's wallet.dat is going to be constantly growing day by day, hour by hour, and will probably need to be replaced with a fresh one periodically, so that the site isn't running off a 100mb wallet file. These are just 'temporary' payment addresses, 99% of which might not have ever even had payments sent to them.
the site in category 2 might have 1000 customers, but it's wallet.dat could have as little as 1000 addresses and not need periodically replacing. Indeed for this kind of site replacing the wallet would be a bad thing, as each customer has been given his 'personal' address for that site.
now you tell me, what would be the best way to run these two types of sites on the same server?