I used to be very quick to upgrade to each new OS release or update, but start to see this more and more of a burden. Sometimes, it is more productive to stick to a certain system, certainly for an older computer (e.g. older PC with XP).
Apple strongly enforces upgrades and they are often cheap or free to begin with. But as I sometimes rely on older, incompatible software, I have Leopard on my old Powerbook, Lion on my iMac, Mountain Lion on my new Macbook Pro (waiting to update) and Mavericks on my old Macbook Pro (for testing now). I'd like them all to be in sync, but then I am enforced to also upgrade Parallels for running on Mavericks (= 50€ upgrade when doing a free OS update…).
For iOS, I stick with the newest, with very little options to go back. The only way seems to be forward, until the hardware is too old.
I can more and more understand people sticking with a stable system and not upgrading anymore.