There are perfectly good reasons for not upgrading, or at least for being scared of upgrading. The typical WordPress installation should be fine. If a plugin doesn't work after an upgrade, then it probably wasn't a very good plugin anyway and so you shouldn't be using it.

I've had serious problems upgrading because of integrations between WordPress and other software. If it's just a WordPress install, then that's fine, there isn't really anything to be worried about, just hit the "upgrade" button and be done with it. But for seriously customised setups you need to be darn careful as a small change in WP can break the software it's interacting with.

I have a rather scary upgrade to do either this evening or tomorrow and I'm NOT looking forward to it.