Of course they don't need to be updated, but when I'm paying $50+/month for essentially a luxury (I could live with a prepaid dumb phone), my expectations go up. I'm paying a lot for fun/interesting technology and I want to partake now in the new whizzy things that people are raving about, not 9 months from now.
What "new whizzy things" are people raving about? I haven't seen anyone get even close to excited over a new OS feature since iOS 4's multitasking -- from the people I know, the response to iCloud, FaceTime, iMessages, Notification Center, ICS's Face Unlock, etc. has been a collective "meh". The only thing I've heard anyone seriously happy about recently was Windows Phone 7.5's ringtone manager.
Doesn't have to be a new OS feature directly. I'd like to run the new Chrome browser but can't. In any case, I'd like to decide "meh" or not for myself.
You're right that most changes have very little impact, so this is largely psychological. It hits in two ways for me: 1) the delay between the product's "release" and when I can get it, 2) the uncertainty of whether an upgrade will even come at all for my phone.
