With a quorum disk, you can in fact have one node left and still have quorum. This is because the quorum drive should have (node-1) votes,thus always giving the last node 50%+1 even with all other nodes being dead.I've never tried testing that use-case extensively, but I suspect that it is only safe to do with SAN-side fencing. Otherwise two nodes could lose contact with each other and still both have access to the SAN and thus both be individually quorate.
I our case, a particular node will run a particular service from a particular directory in the disk. So, even if 2 nodes looses contacts to each other, they should not end up writing or reading from the same files. Am I wrong ?