Giving stuff away without a license can be hazardous to your livelihood. This is due to the concept of "Implied Warranty" (http://en.wikipedia.org/wiki/Implied_warranty), which says that anything you sell must be suitable for its intended purpose. This means that the fruit that you buy at the store cannot be spoiled, and the software that you sell should be free of serious bugs.

"Wait," you're thinking, "I didn't say sell. I said give away!"

Ah, yes, but were you planning on accepting pull requests? Because exchange of money is not the only way to sell something. There is also the concept of "In Kind" payment (http://en.wikipedia.org/wiki/In_kind). So, you make a library with something like a WTFPL license attached. I download your library and use it for my product. Then, to show my gratitude, I submit a pull request with a new feature to one of your other projects (an "In Kind" payment). Later, I find that your first library has a bug that's causing problems for my customers...

...so I sue you for violating the Implied Warranty that you didn't disclaim in your WTFPL license.

This is why all of those open source licenses have the ANNOYING ALL CAPS clauses explicitly disclaiming any implied warranty.

The More You Know!™