Yeah, donating miles to a "generic" charity doesn't sound like a very good idea. FF programs are "complicated," with tons of restrictions, and it would seem very hard for a charity to make good use of them. My guess is that almost any flyertalk member would be able to use those miles for far more value. Both the frequent flyer and the charity would be better off if the frequent flyer donated cash (especially when you consider the charitable tax deduction for cash).

I also have a suspicion -- never investigated -- that the relationship between airlines and charities receiving ff miles could be a bit of a "scam." Do the charities really redeem the miles like ordinary members would, or do the airlines make a charitable contribution from "the money" collected by donated ff miles? It would seem that most charities would have limited use for actual ff miles, given the crazy restrictions attached to them. It would seem far more logical for the airlines to simply donate seats to worthy causes.