Computational power required for a route flap is not the issue here.
Many people have stated that, statistically longer prefixes flap more. Unfortunately, they have then said that because of this shorter prefixes should have looser dampening parameters put on them, when what they really meant was that the longer prefixes should have more strict dampening parameters put on them. Yes it is exactly the same thing, but it is an important semantic distinction. If a group of prefixes categorized by a its length tends to flap more than the average, then said group should have more strict dampening parameters placed on it.
Alec
You're right - what you propose makes some sense. The reason people have proposed and are damnening on longer prefixes is: 1) To encourage people to renumber into larger (P and/or PI) space, and 2) To lessen the percentage of the net which will be temporarily unreachable by the aggressive dampener. Avi