People who declare their work to be in the public domain are doing that because of the simplicity. If you include an all-permissive license and the public domain declaration, it's no longer simple and it's not as simple as an all-permissive license only.
By the way, I completely forgot to mention another reason for avoiding a public domain declaration, and going with a license: namely, liability!
The liability disclaimers in, for instance, the BSD license are important.
In fact, I have come up with a slightly modified BSD license.
My version makes it clear that retaining the license is a condition of redistribution, and that use of the software is subject to agreeing with the disclaimers.
The original BSD license lumps redistribution and use together, and only stipulates that the license not be removed.
All proprietary software EULA's I've ever seen make it clear that you accept the liability disclaimers as a condition of using the software. (E.g. by clicking on some Agree button, or opening a shrinkwrap or whatever.)
The disclaimer part of a freeware license is in fact a use license; they are not strictly redistribution licenses.
(By the way, I don't think it's a good idea to go around customizing de facto standard licenses willy nilly; I thought long and hard before making the decision to fix the BSD license for myself.)
For what it's worth, as someone who currently favours the public domain over copyright for his work, I will say that it's not especially because I think that it's simpler to choose the public domain; it's because I've tried to understand what copyright means and what public domain means and I think that public domain makes more sense and better serves my interests.
In particular, it reflects the fact that I don't think readers of my work should be obliged to follow any rules I might set (license) when they turn to the creation of derived works. They should go and do as they like and, where appropriate, acknowledge prior art (as a matter of professionalism).
I also feel that the problem of associating each "expression" with it's legal creator is a nasty problem fraught with difficulties; certainly, in this regard, you can say that I choose public domain because of its simplicity.
By the way, djb has argued in defence of the public domain here:
Anyway, I'm interested to read other developers views on this topic but I'm no authority and don't want to give the impression that I think it's unreasonable to choose differently. And anyway, most do! GPL, BSD and the rest seem much more popular than public domain.
