well session man;
If you're a session musician (of the pub not the studio!) you wouldn't be asking that question. You would know that Sidmouth, for example, started as a gathering of like minded musicians and dancers from the EFDSS, and the "make some money from bums on seats" part of the festival is only a commercial add-on. Not unwelcome, because it helps to "spread the gospel", but not essential.
As I said before, we pay enough for accomodation, food and drink, (and incidentally providing entertainment) without buying tickets to see groups we're not always interested in. If the performers are interesting of course we buy event tickets, but personally I've only found one festival where a season ticket was of any value.
Much more important is meeting and playing with old friends, and learning new tunes to take back home.
I appreciate that organising festivals is hard work, and the often provided facilities for youngsters are wonderful for kids and parents, and there are good (sometimes) workshops, for which we pay the same as everyone else who attends.
If it's a folk festival, shouldn't there be room for folk musicians (of the real sort, not the stage ones).
Of course we musicians are a minority, but there is usually a much larger contingent of Morris Dancers, who also come just for the love of it, provide dance displays over several days, and frequently make tremendous efforts to involve the "audience", which is what "folk" is about. All at no expense of money or effort from the festival organisers.