Joe, its the fear of PTFB that the MBC can induce. With a tune it is somewhat allievated as the OL/CL transition is changed. The only reason to get a MBC is to overcome the capabilites of the stock BCS/WG. By using a $35 GMBCS (or an overpriced alternative) you can do the same thing. Try looking at it as a different way to skin a cat.

An AVCR is a different ball of wax, a MBC will not be able to do gear specific boost. Sure a MBC equipped car wil make just as much power on a Dyno in 4th, but how is it gonna react in 1st, 2nd, 3rd, etc....
Thats where the money spent on an AVCR will start paying off (esp on the non-DBW cars). The DBW's have teh capability get around this a little.