Here is a fascinating paper entitled Mediation Providers No Longer Add Value
It started life as a debate in 2006 sponsored by MATA but is now reproduced as a very readable think piece and has input from notables like Richard Schiffer, Chair of ADR Group and Elizabeth Birch, founder of ACI.
".... The question that is asked is whether Mediation Providers add value. So I ask myself: Is this like turkeys voting for Christmas? Are we in competition with each other? Or are Mediation Providers useful to the individual mediators?"