I must be missing the point or something. Why would you move to Multiple Spanning Tree instead of dividing the network at layer 3? As I understand it, the purpose of MST is when you have so many L2 connections, and so many vlans that it becomes necessary to aggregate your instances of STP, and segregate STP domains to prevent unnecessary re-convergence during remote topology changes.
Are there really networks with L2 domains big enough to needs this? Wouldn't an L2 domain large enough to need this already be inefficient?