Valid arguments on both sides.
I see your point BLZ: I've done a few small pro-bono projects and they can quickly get out of hand if you're not careful. (eg, "client" pushes for more revisions, project snow-balls into something bigger, client/project become an all consuming fire that takes over your very soul and eats your paying clients' projects for breakfast, lunch and dinner, etc, etc...)
The "there are no truly self-less acts" argument can be true, if you want to see it that way. I think it comes down to your point of view and your disposition: I wouldn't expect (or want) help from someone who is going to complain about the whole time. But, I also wouldn't want help from someone who does it happily but ONLY so they might get a favor out of it later.
If you wanted to see it that way, then even the person who helps out of the goodness of their heart, just for the warm fuzzies it gives them, is doing it out of selfish desires.
Maybe Shaun(OfTheDead) has it right, though: if a person loved puppies and someone wanted a poster promoting the adoption of puppies, said person would most likely make it, if they loved puppies enough. Same goes with everything: hospitals, politicians, SitePoint forums, lawyers, Satan, churches...
If you believe in it enough, I guess you'd probably support it the best you know how, right?