It’s a balance — sometimes those free sessions actually offer you a lot of value (a new idea, an interesting concept) or the person or company has strategic value to you. In the latter case, one is trading one’s time for something that may pay off well later. I think one should take some of these kinds of calls for a couple of reasons: it’s generally a good stance to be friendly and helpful, there’s always more to learn, and being generous with your time and expertise tends to pay off in the long run.
At the same time, all any of us have is time and our expertise, and we have to be wise about using them to best effects. In my mind, a lot of these meetings are early business development activities. Perhaps a good solution would be to plan out some amount of time every week (or month or whatever) for these kinds of meetings and keep to that.