Most of the time, I end up having to dive solo. When you teach a class, and go to do checkouts, you are solo the whole time. When you DM on a charter, you solo dive. When you volunteer to assist someone with their checkout dives, you are solo. Underwater hunting, mostly solo. Photography, solo. There any many instances where you have to dive solo.
There are other times when you don't have to dive solo, like on a regular charter with no students. Now you have a new issues to deal with. Is it safer to dive alone or with a buddy who is a complete idiot. I would say it's better to dive alone. On random charters, where you don't know anyone, who knows what buddy you'll get. If I don't feel comfortable with a buddy, I'll dive alone instead.
Ideally, I would rather dive with a buddy. However, not many people are good divers. There are maybe 5 people that I would feel comfortable diving with. Someone I could depend on if something went wrong. I can't always dive with them so, I need an alternative. I am not saying that diving solo is preferred, it's not. Having a buddy greatly improves safety...the key is finding a good buddy, a very difficult task.
Regardless of if you solo dive or not, everyone should have the ABILITY and SKILLS to do so if needed.
Solo diving, and diving in general actually, requires absolute perfection in skills, mindset, composure, equiptment, etc. Most people can't do this so, I would not recommend solo diving.
When you solo dive you assume all risks.
I'm comfortable solo diving when necessary or when no other option exists, but a buddy would be better. The problem is finding a good buddy.