Windows 2008 multi-site cluster with even number of nodes, why use File Share Majority instead of Disk Majority?

Question

This Microsoft document says that a multi-site cluster with an even number of nodes (ours has two nodes) should use "Node and File Share Majority".

We have shared storage for the two locations. Why is a File Share superior to a Disk for the quorum?

(The SAN is mirrored in both locations.)