I consider it very important.
Let's put aside the option to work on personal projects during work hours, as others pointed out. That's plain unethical (unless you have permission to do it).
For a developer, it's a perfect occasion to learn something new. Found a new framework or technology your company doesn't dare to try? Make something good with it, maybe the company will be more willing if its employees have experience with it.
For companies, the reason may be the same. You take the new technology, build something not-so-important with it, and who knows? Maybe you can make some extra income, or it even becomes the Next Big Thing™.
All in all, if you have the time and enthusiasm (and budget, in some cases), go and do it. You can only profit from it (hint: learning that a give technology is not for you is also an important lesson).