I've only done one clean install in 3 years, and that was by choice just to see what would happen. It was to transfer the OS to a new bigger drive. I used Migration Assistant to transfer all the non-OS stuff over, and it transferred everything except USB Overdrive, which had installed part of itself in the System or Library folders.
One thing that might make a clean install faster is that is doesn't have megabytes of cache. With the faster ISP speeds, it will be more and more common that re-loading web pages from the net could be faster than searching through a gigantic cache.
I have only done one reinstall to completely wipe my drive to start "fresh" and that was to get rid of McAfee antivirus after installing it on Tiger. Ever since then I have not had any anti-anything on my iBook. The other times I did a reinstall was to get rid of System 9 (iBook w/Panther is my first Mac 11/8/2003), when I installed Tiger, and when I repartitioned my drive so I could install OpenBSD and when I got rid of OpenBSD. Ditto for Gentoo.
So then you wouldn't recommend a reinstall? My powerbook G4, running Tiger, seems very sluggish lately, especially Spotlight. I installed and tried Quicksilver, but deleted it because I couldn't get it to catalogue my hard drive properly. When I tried to add a folder to the catalogue it would be very buggy, only working less than half the time. I have repaired disk permissions. Could a reinstall be the answer?
clear out the cache and run the daily, weekly and monthly jobs using mac janitor. a reinstall is not going to help a macintosh as much as it helps a DLL hexed Windows machine.
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One thing that might make a clean install faster is that is doesn't have megabytes of cache. With the faster ISP speeds, it will be more and more common that re-loading web pages from the net could be faster than searching through a gigantic cache.