I've seen hundreds that had registry-related performance issues, but most of those were running XP or Vista. It's been a while, since I switched to Macs over 4 years ago.
Now, warranted I agree that most users become obsessive with "maintenance" apps for their Macs... Some are actually warranted with some users.
If you are constantly working on media files and etc - proper defraging from time to time can yield positive benefits due to large files being utilized - moved, deleted and etc.
Again, not refuting your point. Most users don't need it at all. However, there is always that "%" who can see benefits.