You are quite right Busy Lizzie, seeds sown by festivals do indeed escape to the wild from time to time. However, it works in the other direction too, fringe type things incorporate into central positions in festivals. Also fringe events frequently set up in a quite independent way, often in actual competition.
Another example of a fringe event: in the case of Middlewich, for example, it should be remembered that boatie/folkie get-togethers were a well-established part of the Middlewich social scene before MFAB go started. Also that one of the most fondly remembered of the Middlewich festivals was the one that never was: 2001 I think. It was cancelled because of foot and mouth fears(quite reasonably), but it happened anyway to great enjoyment. Those who had always supported the festival just went ahead (the disease fears had by then receded) and got together, the boaties and musicians came together anyway, and a good time was had by all.