Can someone else weigh in on this? I was under the impression that a clean install isn't important, due to the lack of the dreaded Windows Registry in OS X. That's really the root of all evil (or a helluva lot of it) and a major cause for the gradual slowdown and degradation of Windows performance, hence the need for periodic clean installs and definitely for a clean install when upgrading. But OS X? No such beast, so I ASSumed we were in the clear. Or not?
I always find it funny how some people assume the registry is pure evil and it's only function is to slow the system down. Come on, it's just a way of storing a bunch of properties. It can cause problems sometimes, but it's not the devil. All systems get slower with time. We install, uninstall and reinstall apps, stuff inevitably gets left behind, drives become fragmented, there's a billion of reasons for general slowness. Some OSes might manage stuff in better ways and be more resistive, some maintenance tools speed things up, but in the long run, a reinstall will give you the biggest improvement.
I upgraded my Tiger install to Leopard and when it seemed slightly choppy, I did a clean install couple of days later - the difference was huge. All that despite the fact I use Onyx and do my maintenance periodically. Also, when I purchased a new hard drive and reinstalled OS X, I noticed more smoothness. I'm not saying it's as bad as Windows (with inexperienced users, they get dirty very very fast), I'm just saying it's not magical and bulletproof, it just takes more time and the effect isn't as big.
In the case that something goes wrong during an incremental upgrade, you still have the option of doing a clean install and end up no worse than the user who made the radical choice his first choice.
Paradoxically funny, how you mention hours wasted as the major reason in favour of upgrades, but then when an upgrade goes wrong, it's suddenly not any worse then a clean install, although you wasted a lot more time with it (depending on where it gets problematic).