Great Thread / Subject.
The Fringe is definitely important and I largely share Leadfinger's views.
The difficulty is that there are Town-based Fests, and Marquee-based Fests (the latter which I don't care for) and both generally have a different sort of Fringe.I write from Town-based point of view,and as for whether there should be a contribution in sessions is difficult. Many times musicians are populating pubs to breaking point - many of which would be normally quiet-ish. So pubs , eating houses and all the other places we spend our money benefit from our presence in the Town.
It's fine if there's a collection for local causes ,i.e. lifeboats etc., but whether we should subsidise the Main Events is a question I shall leave you to ponder.