Really? Hmm, I've been upgrading the same installation since 10.04 or something, it's been working well. However, I did create a list of scripts at some point to make reinstallation a one-command affair:
How would it "stop working"? I've done re-installations here and there, but I can't think of a single time that it just "stopped working". And yes, after a couple months I usually do an in place upgrade for my laptops, desktop, and servers that have all been running Ubuntu for a few years.
Well, usually everything breaks, rendering the system unusable, requiring a reinstall. And than there was that time where the new kernel (somewhere in 3.x, IIRC) refused to boot on my setup (I'm running Intel x86-64, so nothing uncommon). But that only happened once.