You lose all control over the material and some ugly things can happen.
Either give some example and reference for the above quote, or I call FUD.
Just to be clear, I'm not arguing against the value of the Public Domain and I would be perfectly happy with a 15 year copyright after which things naturally fall into the Public Domain. I was only referring to the OPs question about just declaring the thesis Public Domain and calling it good. CC is a way of "dedicating works to the public domain"
I was mostly referring to the same confusion over rights that the OP was trying to avoid and that was described in the Wikipedia link posted above. There's no clear legal definition of "Public Domain," and there are places where it's actually not possible to legally release all rights (Germany is cited in the Wiki article, but Portugal is also an example, I believe). Those are some of the problems CC was created to solve. On the other hand, CC licenses are very explicit about what they allow and make it easy for someone to reuse the work. Without those assurances someone might reuse a work only to find that they original rights owner (or more likely an heir or assignee) is suing them and there's no recourse because the release into the Public Domain wasn't legally valid. You might still win, but it's murky enough in some cases to cost money defending the case.
There are other things that could happen without much legal recourse if you do manage to give up all rights, for instance someone could change the author's name only and republish it, which, depending on the work and the effort involved my be disappointing. Remember that the OP wanted to make sure that his work was properly cited. Worse, someone might leave the name on it, change the content to meet some agenda of their own and republish it, so that it appears the original author said things they didn't. If I wanted to release something directly into the public domain, I would be inclined to do it anonymously to avoid that problem.
Another advantage to using a real license of any sort is that somewhere an attorney was involved. From your comments, you are not an attorney. Neither am I. For good or ill the law doesn't work the way an engineer would design it. Public Domain doesn't work just because you or I may say it should. It works when and if a court will find that works in most cases. For most of my own stuff, CC works great because I don't want to have to reinvent all of this every time I publish something.
You seem to have a strong opinion about this. Do you have a problem with CC licenses that releasing material directly into the Public Domain seems to fix?