If connectivity with a provider is lost because of a network partition, then "automatic failover" can just compound the problem
Typically, a particular machine cannot distinguish between losing contact with a peer because that peer crashed, or because the network link has failed
If a network is partitioned and multiple clients start writing to each of the "masters" then reconciliation will be a pain; it may be best to simply deny writes to the clients that are partitioned from the single provider
but the arguments against mirror mode were more semantics (e.g. "MirrorMode is not what is termed as a Multi-Master solution" and "MirrorMode can be termed as Active-Active Hot-Standby") rather than any real negatives.
I'm essentially looking to have two LDAP servers and keep them in sync. LDAP consumers will be configured to query both and the web interfaces would be configured to talk to their "local" instance with DNS pointing at a preferred instance.
For me, the biggest concern I have about implementing MMR - plain or mirror mode - is the challenge of recovering from a problem. Mirror mode seems to be simpler in that respect because only one node has the writes and therefore reconciliation should be straightforward.