You have to wonder how effective a DM can be helping one diver in a group of 6 he/she is with underwater. This ties in somewhat with the accidents that occur during Discover Scuba dives where uncertified divers can be at a 4:1 ratio to the instructor.
I'm going to argue that a DM guiding a group of 6 certified divers is nothing like an instructor with 4 students on a DSD dive. The difference is that if the DM shouldn't have to assist anyone: he should be guiding three pairs of divers, and each diver should rely on their buddy for assistance with minor problems.
The role of a guiding DM is frequently misunderstood. They're not there to re-write the rules that are taught in OW class. They're not there to take the place of a diver's buddy. They're not there to compensate for a diver's diminished skills. They're not there to relieve divers of their responsibility to make proper predive safety checks and plan their dive within their personal limits. They're not there to monitor the diver's depth, air and NDL for them.
The purpose of the DM guide is to ensure people don't touch stuff, and to greatly improve the chance that everyone will find their way back to the boat (which is important for the shop, if the boats are on a tight schedule.)