It's hardly rare. I see people sitting on benches all the time in my zoos. And when it's a larger group (which is a lot of the groups) the bench will be fully occupied. I have started putting benches side by side with a trash can in between so that an entire large group can sit down at once.
If you're zoo is small they won't need them because they don't have enough time to get tired but in the larger zoos it's rare for me to have a camera angle where there is
not at least one or two benches with people on it.
Edit: I just looked around my zoo and I found 4 places where benches were fully occupied, in my newer zoo with only 2000 visitors no less. It's just that they don't stay sitting very long so we don't notice it most of the time. Here's two of 'em so you don't think I'm fibbing: