> But other than that, I can't see a reason why not to have one.

you are assuming that

* they want to write code outside of working hours

* they will put the same level of attention into the code they write outside of work as they do the code they write at work

* they want the code they write outside of work to be open source

failing to do any or all of these things, or doing any or all of these things, should not affect a candidates impression on you.