Okay, I don't get it. I've never hired a guide, so bear with me here, because I guess I don't understand the whole concept.
You're basically hiring someone to take you out on the water (in his boat I assume?) and locate fishing hotspots. This person is probably a local, or at least knows the location well. Not only is this person supposed to know where the fish are, but has some working knowledge of what kind of baits would be successful in said location.
So....how is the guide supposed to know where the fish are unless he goes fishing?
I'm sure he has several locations that any guide with fishing knowledge would have, and a "home field" advantage as well. But how is he going to know what works at what doesn't unless he's out there doing it too? Personally, I would WANT him to be fishing. I would hope everyone catches fish, but if either my party or myself isn't catching anything, and he is...that he'd be able to tell us what's working.
To answer the original question, I would hope the guy would ask if we minded if he fished along with us. Just seems like the professional and courteous thing to do.