I think MS soccer is a great idea under the right circumstances.
Lil' Daddy played school basketball last year....and I'd guesstimate that they cut at least 1/2 the kids who tried out. Probably more 60% got cut.
When school soccer programs start approaching that rarified air....where you are cutting half the kids trying to play varsity or JV soccer....then its time to put a middle school program in place, so that you encourage more kids to keep playing soccer.
But when your "washout" of the varsity/jv process is a handful of kids....middle school IMO ceases to be a need and in fact, weakens development by over-diluting the player pool. Not to mention the coaching pool.