Related to the cost issue, but not quite the same, there's also an issue of trust. When everyone can see the source-code, there's less chance that it contains nasty surprises, and more chance that it was written with your, the user's, best interests at heart, rather than those interests being in conflict with other interests such as the authors' interest in making money from you. Or other, more sinister, interests such as phishing, attacking competing products, or conniving with intelligence agencies.

Which is not, of course, to imply that all providers of closed-source software do those things. But without the source-code, you never know for sure. (Open-source projects, such as Firefox and now Songbird, where you're very strongly encouraged, using trademark law, to use their binary builds, lose much of that trust.)

Peter