That’s an easy one I’m sure. On the other hand, being the only CCR diver in a trip of Open Circuit divers IS diving solo. Generally on every dive but particularly as a trip progresses, nitrogen starts to build up.
In that scenario you will find your “buddies” can’t make the same dive profiles as you do and are in completely different gas loading schedules.
People can argue as long as you maintain a profile that respects the OC “handicaps” you can still respond to an emergency and that’s true. On the other hand, what if YOU need help?
A second important reason is knowledge. It is very unlikely to have an Open Circuit Diver who is knowledgeable enough on rebreather diving to perform (and cross-check with you) pre dive safety procedures and in water checks. In the event of a rebreather related rescue scenario having a closed circuit trained buddy does make a big difference.
One of the reasons why Galapagos Rebreathers exists is to provide appropriate rebreather travel opportunities to the Galapagos. Being in a group of CCR divers here provides several benefits; of course one of them is the inherent rebreather safety that comes from a closed circuit oriented operation.
But this is not the only reason.
Sharing the trip with other CCR divers will allow appropriate rebreather profiles, maximizing enjoyment. A huge factor for diving rebreathers here is that sharks, particularly hammer-heads, seem to get closer and allow longer, more natural interactions when they encounter silent divers. Truth is you can’t have this if there are another 15 open circuit divers in the water at the same time. It’s just too noisy.