I know some of the musicians at both World and Land, and the guys at world are happy with the deal. Regular gig for some specialized musicians.
How much call is there for a professional marching band, or an individual banjo picker outside the amusement park industry? Not so much, and especially when it comes to artists outside mainstream commercial music, there are a number of festivals to play at- but the fees musicians make are going down every year. It's hard for producers of festivals, concerts, etc, to draw at a premium, the audience of the past.
I'm glad Disney and the AFM could agree.
The question might actually be how much would the parks be different WITHOUT live music?