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First of all, I believe rollbacks already revert all edits by the same user. But as I explain below, it's easy to bypass.
I'd just like to say this could be fixed by consulting the history tab. I don't like or use the (diff) link at all. But this takes more time and resources, and is unmanageable if you need to check more a lot of revisions.
Sum of diffs for one user would only partially solve the problem. The vandal would simply create two accounts, one of them vandalising and one of them innocent. With a little extra editing, the correlation could be made to be as cloudy as possible: valid editor in the wrong places at the wrong times, or vandal's assistant making good edits only? And if the vandal's assistant is indeed making /good/ edits, would there be any way to block them? (Someone's bound to make a mistake sooner or later).
I would push for the implementation of an "All Changes Since Last View" option for users. By encouraging users to check the last twenty edits, rather than just the last edit, they click the link, as usual, but instead see a comprehensive profile of changes and can selectively revert if necessary. Multiple diffs on one page would be nice too.
Selective reversion is a time-consuming process though, and would not work for RC-patrol.
This is from a social perspective. Technically speaking, I'm sure all of these features have been filed in Bugzilla, and that all of them have technical problems that make them undesirable performance wise.