I use it because it's free, in active development, and pretty much works well...... and how many closed-source projects meet the same criteria?
A few loss leaders here and there, I suppose: Flash, a few web browsers. Though John Gruber, among others, might dispute the statement that Flash "works well": http://daringfireball.net/2010/01/tablet_musings
Although I do wonder how many people actively go through open source projects looking at code
Not that many people, really. Just the talented open-source developers, the ones who do important foundational work. Or the ones who want to become talented, and perhaps someday accomplish some significant foundational work. Or the ones who want to accomplish something that doesn't come right out of a can, which includes the majority of the Drupal and Rails programmers that I've met. Still, a small fraction of humanity.
But that misses the point. In an ideal world, nobody would ever need to read the code for open source projects: The code would work 100% of the time, and would do just what you want, and would have comprehensive and readable documentation. Even now, the open-source code I use works an astonishing percentage of the time. The point is that you don't want the one occasion when it stops working to grind your project to a halt for two years while you lobby the manufacturer for a new release, or beg for explanations of obscure algorithms, or pay thousands of dollars for a proprietary training course, or prostrate yourself before the author on the internet and beg for some attention to your particular problems:
http://news.ycombinator.com/item?id=1034584
It's true that, paradoxically, people who work with open source tools don't spend a lot of time using or thinking about their ability to read the source code. They don't have to. On the rare occasions when they have to use the source, they just do it and move on, with relatively little loss of time and no worries.