It sounds nice, but you have to take into account the other benefits that mirroring gives you - automatic failover primarily. How would you handle the conversations between multiple mirrors and the witness to ensure that each database is in the correct state? It's already somewhat complicated to determine what will happen to each database if connections drop, that complexity would grow exponentially given more servers.

If you are talking asynchronous mirroring then perhaps those issues go away - is there really that much benefit over replication or log shipping though?