Hammeron wrote
If you do something well NEVER do it for free.
Not sure I agree, though I am not saying that musicians, or anybody else, should just give their skills away. There's a nice story at the moment about a bunch of doctors who have been up in the N Cape in a community where there is a high incidence of colon cancer. The program is partly funded, but several of the doctors and specialist nurses involved did unpaid work.
Some good lawyers do pro bono work - and more strength to them for it.
I know that there are pro musicians on this forum who do fund-raisers for no payment.
What I resist is the presumption that something should be free. It's best summed up by attitude towards "sharing" of music - namely that if the artist wants to give a work away for free then that's OK but it must be the artist's decision, not somebody else deciding that it's OK on whatever grounds are convenient for them.