Any company that leverages open source software should make an effort to donate back to the community, even if the core of their software is closed source. Use Linux? Python? Emacs? Firefox?
Allowing your employees to donate some time towards open source projects is probably a good way to attract talent. Advertising for "rockstar ninja" positions is not.
While I agree that it's a much better way to attract talent, I'm going to say (as an OSS contributor myself) that unless it's already significantly into its funding cycle, a startup where you've got time to spend on random OSS stuff that doesn't directly benefit the company is probably focusing on the wrong things.
A startup where the co-founders have time to work on random OSS stuff is probably working on the wrong things. A startup where employees can spend 20% of their time hacking on OSS or side projects is likely doing just fine. I would be more worried about a startup that expects or demands that employees work 40+ hours. It either means that it's being run by suit monkeys that don't understand how programmers work or has poor project management that requires a constant crunch mode.
Allowing your employees to donate some time towards open source projects is probably a good way to attract talent. Advertising for "rockstar ninja" positions is not.