The only time I really ever had an effect was when my buddy (Brian) and I were leading a group on an evening dive at gilboa. It was a group of three advanced students doing there second night dive making the total in the group 5 divers. During the dive (about 15 min. in), we crossed another group (of more than 2), and I all the sudden started counting 6. I looked at Brian, he too was confused....we confirmed that we were each counting 6 divers and continued on...no-one was low on air, no equipment problems, so we just assumed we missed someone at the beginning....bad leader.
Anyway, we surface as a group when the "6th" diver got down to 700 psi (the rest of us had 1000 to 1500), and it had been about another 20 minutes. The 6th diver then realizes he isn't with his original group....he's been down for a total of 54 minutes. Hmmm...we wonder, where could his group be....nope, they weren't in the water, nope they weren't on the floating dock looking for a lost diver.......they were packing their gear in the back of their trucks and getting ready to head home. We conveniently told them that we had a missing diver from their group, and that's when they decided to do a head count. They had been out of the water for apporx. 15 minutes. Brian and I have made it a point never to dive with that particular shop...ever (we never have, but I'll never forget the name of the shop/club)
Getting back to mike's question, I think the number of accidents would definetly rise, but who knows by how much...there are lots of "periodical" divers out there, and I believe that's where a good number of the incidents lie anyway....why do you think we have the DMs and Instructors to run trips and organize everything for the general diver.