This thread touches on the same question.
Many times a direct 3NT overcall of a 3-level preempt must be attempted as the best percentage action over the long run, but will have a flaw. The 3NT bidder might be considering a run-out if doubled. Partner's redouble (not always successful, of course) should be "It's O.K., stick with it." The rewinder will probably have some nice fillers to make 3NT a good deal.
A few times over the years we have encountered a double of the 3NT overcall which was tactical in nature to get us off our good spot. I am not saying we should be looking for this to happen again, but it might be another reason for the redouble to be to play.
BTW, I do not agree that 3NT is typically bid with a long suit and a stop. Certainly that is one possibility, though. Much more common would be a balanced 18 or so.