Good point StrateandLevel (although I suppose one could train in one's own airplane and get liability-only insurance through a group plan, some of which do not care what the pilot's experience is).
I can't see any advantages to ab initio instruction in a multi-engine environment. No sensible pilot would dispute that this would add tremendously to a student's workload, and require a significantly longer period of instruction than would be necessary for a single engine PPL. The cost would almost certainly be more than a multi rating piggy-backed onto a single engine license in the usual manner.