I think that the answer to your original query has to be in two parts:
1) For those of us who have regularly-updated effective anti-virus programs, it is over the top and you can safely uncheck the box.
2) Judging from the frequency of requests on this site (and no doubt others) about help removing viruses, obviously a lot of people either do not have AV or "fit and forget". For them it is a sensible security measure.
However, it might have helped if MS had posted a clear warning with SP1.
This thread is now locked and can not be replied to.