The only problem with this is that you must sign in to the account that purchased an App in order to update it.
It's a real pain doing that on the iPhone itself - but doing it in iTunes isn't too bad.
When the iPhone prompts you to do an update it will give you an input box for the password belonging to the account the original file was downloaded under. It doesn't seem like a pain on the iPhone. For example, if my girlfriend wanted to update an app from me, the box would prompt her for flipside186@example.com's password and not her own.
I think it's easier on the phone, truthfully.
With Apple's new changes, that will usually bring up a prompt for the credit card information associated with the other account.
If you're sharing an account with someone else, it's not likely that you'll know that information.
It does bring up a prompt for CC info and even tho I know the security code on my husbands CC he used, it wouldn't work, it would lag, then cycle back to my itunes account ask for PW, lag, ask for his PW, then security code, then repeat.