This is a contentious issue, but I candidly admit that just over one-third of my dives are solo, primarily because I enjoy underwater photography and that would burden another diver who doesn't share that priority. My U/W photo dives are simple matters of descent-photos-ascent in an uncomplicated "bathtub dive" mode (no technical or penetration diving when I'm solo). I seldom log solo dives unless it has been photographically productive. Do I recommend solo diving? No, I don't, primarily because lapses in judgment are epidemic.
I have been in "babysitting" situations, and it has affected the quality of my dive experience. For example, my dive buddy is Basic Open Water certified, and his depth limit is 60 feet. My depth limit by training is 130 feet, but it would be irresponsible and unethical of me to leave my dive buddy, so his limit becomes my limit.
Some novice divers (and a few not-so-inexperienced divers) are "air hogs"; they can empty an air cylinder very rapidly by breathing like a racehorse. When they run out of air, our responsibility is to ascend together and must accompany that diver to the boat or shore for safety's sake. However, when I check my remaining air and have >1,000 psi but my buddy is down to 500 psi, I'm aware that air consumption cuts my dive short.
No, in all candor, I do dive solo from time to time. I address my own priorities and function at a very relaxed pace, and I ascend with 500 psi or more in my cylinder. While below, I wait patiently for the photo perspective I want, usually in relatively shallow water, since I want the light penetration for good photos. My dive profiles are very conservative.
To summarize, when you dive with a buddy, his/her safety is the higher priority. When I dive solo, I operate responsibly. I would NOT recommend solo diving for divers who don't have experience, confidence or total familiarity with their equipment.