Earlier in my career I was the president of a local (San Francisco) professional association. I have many fond memories of people, events, and overall personal growth I experienced during that service. I also can't forgot a comment made by one of my board members.
During a particularly vibrant discussion, that individual responded, "but it's only a volunteer organization!" In other words, as I interpreted the remark, we don't have to give it our best and/or exert our full energies because a) we're not serving an paying employer or (in his case) a client and, as a result, b) there's very little at stake.
Granted, earning a living ranks high on the list of life's priorities. And we certainly need to prioritize our commitments. But to argue that we should lend anything less than our total energy and talent to any commitment not only doesn't serve the organization or cause we've pledged to help, but it reflects poorly on us as individuals as well.
In other words, how you do anything in any context (paid or unpaid) is indicative of how you do everything. So if you're telling me that you'll hold back on your effort because you're volunteering your time, I'm wondering how you're going to perform for a paying client--and I'll hesitate to refer one to you.
Moreover, I've been around long enough--and have been involved in more organizations than I can count--where the pervasiveness of such thinking doomed many groups to periods of stagnancy, if not their outright demise. That is, what professional associations and similar entities can't survive if members don't step forward to lend their time and talent.
The bottom line here is to put forth your best effort no matter what the circumstances. If you've made a commitment to a group, meet it. Negotiate what you're willing to do (or not do) up front. Clear work boundaries are the sign of a true professional. The perceived need to hold back and/or cut corners in terms of effort and energy because an organization you've committed to serve isn't paying you, in my opinion, is anything but professional.