My (obtuse?) point is that you are either a dive buddy pair or you are a solo diver. There is no middle ground. Kind of like Pregnant or NOT Pregnant?
I'm gonna disagree with this one; there are shades of grey.
On a true solo dive, you are alone. In the event of an emergency at depth, the odds of someone coming to your assistance are one in...none, pretty much.
If you are diving as part of a loosely affiliated group following a guide in high-viz., excellent tropical conditions as is common in some tourist diving destinations, in the event of an emergency, the odds that someone will see you and help are better than that. And the odds that in a simple OOA situation you can fin over, get someone's attention and get assistance may not be bad.
Similarly, if there's 1 chance in 4 a 'bad buddy' will render effective aid on an OOA emergency, that's 1 more chance in 4 vs. a true solo dive.
There is some middle ground, where situational resources at hand fall somewhere in between.
One question is what you expect out of the buddy system:
1.) Remain in immediate proximity, frequent visualization of each other, noticing nuances of each other's behavior, like having your own personal rescue diver ready to intercede if needed.
2.) Same area, could be 15 feet apart, looking for each other occasionally, generally remaining in the same area, if someone goes OOA fin over and indicate the need; if you were to have a seizure or something, well, you're probably dead... Basically, the buddy is viewed as a redundant gas source swimming in the area.
1 Is the politically correct answer here; 2 I suspect is more common in many places.
You won't get much more agreement on how to handle solo. The politically correct answer is the full setup recommended in class, a redundant gas supply (e.g.: pony) with enough gas to make it back from the farther planned point of the dive to point of origin at the surface using a rule of 3rds, and plan the dive around getting back with 1/3rd remaining in primary gas supply. Yet some people post to the effect dives that aren't too deep, in their view, they can CESA if things go bad and they may not pack a redundant gas source.
The Spare Air is sort of a middle ground there.
Richard.