I rather suspect that this is because people are in that awkward situation where they vaguely know a little about the subject but not enough to make fools of themselves, Greg.
I vaguely remember being told how wonderful reg cleaners can be by someone that was fairly computer savvy some years ago, and played with the one he gave me for a while, then gradually forgot about it. Did it really achieve anything? Don't know.
I do know that there is at least one, and probably a whole stack of phoney registry cleaners that are nasty as you like. Spybot picks these fake registry cleaners off people's machines regularly when I'm doing the eternal spyware and virus removal jobs. Can't remember their names, alas - I just trust the usual tools (Spybot, Ad-aware, Hijackthis, Ewido) to get stuff right.
So don't just download and hope for the best!
If you are going to try one, do a good solid Google search first and make sure it is legit. Be sure you are getting good info by notiicing which site says Program A or Program B is fake or legit. (E.g., if Castlecops says it's dodgy, don't touch it with a bargepole. If they say it's OK, you can be pretty confident.)
But will it do any good?
Hmmmm ..... probably about as much as a Granite Digital cable on your CD-ROM drive.
In reality, I suspect that the performance benefits to be gained are minimal to nil (guess only - no certain knowledge) and the real registry nasties are dealt with much more effectively by Spybot & etc.
But, as I said, none of this is from certain knowledge (apart from the known bad egg stuff). Maybe someone will drop by and fill us in with something a bit more concrete.