The simple and fairly complete answer is choice. The thing that really appeals to me about the open source software movement is the fact that if I don't like the direction my distribution is moving, I can change to another without loosing out on much functionality. If you don't like the way your IM app works, try another one, or write your own. If you don't like something you are free to change it.
I was reading a posting on the infamous slash dot on a comparison between Snow Leopard, Windows 7, and Ubuntu 9.10 (http://tech.slashdot.org/story/09/08/31/1916238/OS-Performance-mdash-Snow-Leopard-Windows-7-and-Ubuntu-910) and came across a great comment from user KingSkippus (http://slashdot.org/~KingSkippus) which nails the issue for me, I've posted the whole comment because finding it over there might be challenging, and it really is a great post from that site.
99.997% of the people using these computers don't care.
First of all, I think that number is way too high. While it may seem that way sometimes, people do care. Maybe not even a majority of them, but enough that it does make a difference.
Second of all, those who in theory don't care, when explained why it's important, start to care. When you add up the cost of upgrading from Windows 95 to Windows XP to Windows Vista to Windows 7, along with all of its associated applications (I'm looking at you, Microsoft Office), versus the cost of upgrading through the various versions of Ubuntu or any of the other popular distributions and their associated applications, people really start to notice. One of my favorite things to do when I'm showing off Ubuntu to people is to open the package manager application. I tell them it's like the "Add or Remove Programs" applet, except that you can actually add programs. "All this stuff is available to you for no cost. Just click it, and you're good to go."
When you explain to these people how there is absolutely zero technical reason why they can't have a movie or song play on the DVD player in their living room, their iPod, their computer, and anywhere else (and anyway else) they want to play it, but that thanks to DRM systems incorporated into Windows 7 and Mac OS X, they are artificially restricted from doing so because some third party has decided to "manage their digital rights" for them, it definitely gets their attention.
When you explain to these people how honest competition from really smart people doing really smart things just because they can and because they feel that others should benefit from their collective knowledge is one of the reasons why a lot of commercial closed-source software these days that might otherwise cost hundreds or thousands of dollars is sold for really low cost or given away for free because of how hard it is to compete with volunteer work, it also gets their attention.
When I show people my web browser (Firefox with AdBlock) and how I don't see particularly onerous ads on web sites because the person who wrote my browser isn't beholden to financial interest or corporate mandates, it has raised a lot of eyebrows.
I could go on, but hopefully you see my point. Free and open source software benefits everyone, even people who don't otherwise care, even people who shun it in favor of commercial and/or closed-source options. And sitting back and saying that people don't care isn't very productive. It's in our best interest to actually educate people so that they will care.