The essential point of MESI is that it doesn't need a single master. In a sense, anyone who has exclusive ownership of a cache line is the "master" for that one cache line.

The downsides of MESI are that (a) it requires broadcasts, which don't scale very well; and (b) it doesn't tolerate partitions -- which also imposes an effective scaling limit, since large systems are always partitioned (usually with a partition of N-k and k partitions of 1, due to k nodes having failed).