These people, even if they know that upgrade is available won't bother. The existing device with it's current firmware works for them, so why change it.
1) Developers must invest more time and money in order to support a wider range of firmware revisions, as the user base is more fractured. This leads to less app selection for users, and more buggy behavior on apps that are available. If you're an iOS developer, you can focus on just the last major firmware revision and reach 95%+ of the market, or support the last two major firmware revisions and reach 99%+ of the market, whereas on Android, you're forced to support a few more than that, and the populations stuck at each older revision are much more significant. Read the article for Android stats by revision. Here's an article with a bit more detail on the iOS side:
http://www.marco.org/2011/08/13/instapaper-ios-device-and-ve...
2) Performance and security updates are unavailable, making the user experience sub-optimal, and potentially putting users' personal information at risk.
Users are missing security fixes, developers can't use new features in new versions.
And installing stuff on a PC is ingrained into people's psyche, if they can't do it themselves, they get some other folks to do it for them. Upgrading phones? Not so much. My friend never updated her iPhone 4 once. She once used iTunes to transfer songs to it when she bought it and then hated iTunes and didn't listen to music on the phone enough to connect it again. I installed a bunch of apps/games on her phone that she uses/plays, but whenever I see the phone, I see updates to the apps that are never installed from the app store. The other day, 34 apps were pending install on the app store icon!
I know that's unfathomable to us geeks, but that is how much of the general populace is. They wouldn't even know what a ROM means or how to go about finding what's the best one from xda-dev or how or even why they should attempt to install it instead of just using something that "works"(well or not) for now.