I would be careful calling it the "least encumbering"- there are definitely shorter, easier to understand licenses with fewer restrictions (i.e. zero).
The main problem with the public domain is that there is no such thing as "the public domain". Each country has its own copyright law, with its own terms and conditions. What the public domain means in the US is not what the public domain means elsewhere i.e. Germany. In Germany an author has Moral Rights which the author is incapable of giving up and may only transfer as apart of their will. So rather than require all German contributors to be dead and have a will that grants their moral rights to the project, using a permissive license gets the same effect.
SQLite ran into trouble by using public domain, due to it not being recognized and for other reasons, and as a result will sell you a copy under a different license that guarantees your rights.
Several European countries do not recognize the ability of copyright holders to completely give up their copyright. So it needs a license of some kind.