Why indeed.
I think this stems from a misunderstanding on how system restore works. Some, mistakenly, believe SR is automatic. That is, it will replace files it "sees" are missing without user intervention. Not true. The only automatic thing about SR is it will create new restore points automatically.
To continue along the misconception. When cleaning a computer, sometimes the malware returns. Again the belief is that it is coming from the SR. Again not true, it's most likely a malware process replacing/creating the bad files. So SR gets turned off, all malware finally is removed and everyone gets a pat on the back, not realizing how close to total disaster they were.
With SR turned off and the wrong file is removed, computer doesn't boot. Now what?
At least an infected restore point is better than nothing. The key thing to remember is, the SR points will do nothing until you use them.
Cleaning the SR is a simple task. Once the computer is deemed clean, then turn off SR and turn it on. No need to reboot with xp (vista too, i think).