In reality, what happens when someones cleans out their registry? Ok, you just made windows lose all memory of that adobe photoshop trial version you had installed a year ago. No loss or gain of speed by getting that out of there.
Ultimately, cleaning the registry is only handy for those who are actually having problems because of it. If you run a tight computer like i do, then you're unlikely to have any problems arising from the registry. Then again so called "computer experts" who run windows and claim they run a "tight computer" usually do dumb things like running as administrator all the goddamn time; for them taking a look at the registry is perhaps a good idea (a lot of these "computer experts" also don't know how to diagnose problems well at all).
What's the other reason for doing it? Not much of one, unless you just feel like having no more entries of previously installed software gone for ****s and giggles; or if you're a little dumber, then you get the placebo affect.
Ultimately, this topic has been a great exercise of thought on such mediocrity as the windows registry. We've all exchanged ideas and furthered one very good argument. But, now i see it going towards the area of the dumb side.