First off, I Am Not A Lawyer, and nothing I say constitutes any sort of legal advice. That
said, here's my lay opinion.
1) Lawyers I trust tell me that the whole concept of "releasing into the public domain" is of
doubtful validity in US law. There are various ways spelled out in US law that a copyrighted
work can enter the public domain. One of these is "abandonment" of the copyrighted material.
But it is not clear to these attorneys that you can *explicitly* abandon a copyrighted work.
2) One of the rights protected by copyright is the right of attribution. If I were to take
public domain code and claim to have written it, this would, I believe, be perfectly legal (as
long as it wasn't part of some other actionable offense). If you are proud of anything in
your code, you probably want to avoid that.
3) One of the things a (good) license tries to do is to control legal risks of release of
material. Disclaimers of warranty and liability (for what they're worth) and legal terms of
use can help to protect the author from expensive and potentially damaging litigation.
4) License notices and copyright notices are a way of protecting the user of a copyrighted
work from a malicious author. Under current US and International law, published work defaults
to "copyrighted" even in the absence of any copyright notice. The placement of copyright and
license notices specify exactly which rights are granted by the author, and under what terms.
Anyone who uses code from a large "public domain" release may run a substantial risk that the
author may say "oh, but I didn't mean to release *that*" to a court. I wouldn't want to be
defending myself from a copyright infringement charge over this.
Conversely, if an author publishes copyrighted work related to her previous public domain work
and inadvertently omits the copyright notice, it seems easy for someone unscrupulous to claim
that it was part of the public domain release.
Like I say, I'm not a lawyer. But for all of these reasons, I don't plan to release any more
code for public use without licensing it somehow.