it's not pointless
you can do A scan, and then go through the results, and pick and choose what to delete.
also, if you're adventurous, you can go ahead and let the cleamer delete all of it's findings, and then make sure you keep the backup.
when the registry gets too big, it will slow down, but for most users, this will never happen
so, ya, for most users, a registry cleaner does more harm then good