Doesn't iTunes offer to update your iPhone automatically (whenever the iPhone is connected and there is an update available)? I would guess plenty of people just click OK.
It does, but then it downloads some CD-sized thing, which you can cancel at any point.
I have clicked update once. After slowly fetching the update, the download was corrupt, and iTunes wanted to pull the entire thing down again. Slowly, of course. So I canceled, and haven't accepted any updates since, but iOS 5 might be worth trying again.
Sure some people don't ever connect it, but is it a large percentage? I imagine most people (1) have some digital music on their computers, (2) want to listen to it from their iPhones, (3) use iTunes to copy it over.
Of course someone might (a) not listen to music on their iPhone, or (b) buy music from iTunes directly from the iPhone, or (c) copy music to the iPhone using something other than iTunes; but I imagine none of these things are very common.
I have absolutely no data, I'm just guessing and would love to hear from anyone that does have data about this.
I have clicked update once. After slowly fetching the update, the download was corrupt, and iTunes wanted to pull the entire thing down again. Slowly, of course. So I canceled, and haven't accepted any updates since, but iOS 5 might be worth trying again.