Assuming that everybody has the luxury to have additional office hours available is a bit far from reality in my opinion.
I mean, if you can offer office hours for an open source project you probably are already so popular that you are able to work on it fulltime, right? And if you're not that popular, you cannot offer office hours due to your daytime job; as you would have to decrease the rest of your remaining free time that you probably need to sleep and eat.