Well, first off I'm a full time bassist and I have yet to see a single complaint (like that would actually phaze me). Second, sit ins are great for your musicianship in general. I play open mics all the time and I've learned that being a quick learner is not only fun (because you get to play a lot more), but also is something that gets around- and people start calling you up to ask if you can fill in on such and such a day for their usual guy. Most of all, the more you sit in, the more you become adept at learning things on the fly... learning how to "jam."