But please, write them. A lot.
I've worked on a lot of projects during my life (especially if I count ones I just throw a patch at and never look at again), but one thing that often runs true is that they usually don't have enough unit tests written. This is of perhaps because they don't have enough anal-retentive people like me badgering other people to write unit tests.
Humour aside, unit testing is important, especially when you're working on a library. Bugs are natural, and will happen, but unit tests help to catch bugs (so you don't release horribly broken code), and to prevent the same bugs happening in the future, which is also quite possible.
I was wandering around Qt's code the other week, and was a bit astonished to notice just how few tests some of the base parts of Qt had (QList, QVector, etc.) Now, it's perfectly possible to argue that due to the very fundamental nature of those classes, any breakage would be caught pretty much immediately when compiling Qt itself, but I personally feel it's a lot nicer having proper tests that validate the way your API is supposed to function, *especially* when it's for something that low level and important.
So, I did something about it, and QList and QVector are now fully covered test-wise. Hoorah!
There are still plenty of other areas that could probably use some test loving, so why not check out Qt from git and lend a hand today? :)