So I will ignore the flaming about whether solo is safer or less safe than buddy and just address the original question, or rather statement, of this thread, "NEVER DIVE ALONE!" This is utter nonsense. Clearly, there are appropriate times to dive alone or there would not be solo certifications from PDIC and SDI - two long standing and well respected agencies. Clearly, diving alone is found to be acceptable in some situations by some people who have trained to a level of self-sufficiency that has allowed them to do so comfortable, because people are doing it and succeeding.
I do dive solo, usually in my local quarry where I know the entire place pretty much like the back of my hand and where I have done 1000+ dives over the course of 14 years.
I do dive solo, sometimes on vacation when the plan is simple and straight-forward. For example, in Bermuda, I was matched with a buddy who was 65 or so. He got tired and cold after about 20 minutes and thumbed the dive. Since the dive area was all within 100 yards of the boat and less than 30’ max depth, I chose to swim my buddy back to the ladder and then continue solo for a second 20 minutes. No biggie.
I have, on one occasion, gone solo when I would have preferred to have had my buddy with me, but due to circumstances it was solo or no go. Since I had been on this wreck before a couple times, and once just a couple weeks prior, I decided I would go it alone and just run the same plan we did last time (with buddy) while my buddy stayed topside to babysit his boat. It was close to the edge on my comfort zone, but not over the line, and had I gone outside my comfort zone I could simply begin my exit at any time, since there was nobody else to disappoint by calling it too soon.
All that said, there are also many time I prefer to dive with buddies. I prefer a team of 3, even more than 2, because now there are 2 buddies watching out for each of the 3. I do not think a team of 4 or more is a good idea, because at that point it starts to get unclear who is watching whom. When there are 4, they may all go together, but there should be some agreement that you are really 2 buddy pairs. So when I go on a "herd dive" with a dozen people, it’s either an agreed 4 teams of 3 (or 6 pairs or whatever) or I consider myself to be solo diving really close to my friends (because I have little confidence that anyone would notice right away if they left me behind - and that has happened - in the middle of the dive).
So my preference, especially in unfamiliar surrounding, goes like this:
1)Team of 3 - me and 2 of my regular buddies who are familiar with me and each other
2)Team of 2 - me and one of my regular dive buddies
3)Team of 2 or 3 - me and 1 or 2 divers I know or believe to have decent skills
4)Team of 1 - me (and maybe my scooter) in appropriate redundant kit
5)Team of 2 or 3 - me and random buddy of unknown origin