It is called a Solo Cert and I have been as many Photographers diving solo for years! So the answer is YES! People who scrap boat bottoms do it all the time! On and on!
I think one of the most common fallacies among underwater photographers is that since they don't have the skills to dive with a buddy or a team when shooting, then diving solo with a camera becomes somehow more OK. Diving with a big DSLR rig and shooting macro is not hard at all in a team, it just requires that you've got your other dive skills together and not some insta-buddy who doesn't have any interest in the activity.
I've actually seen people saying "the moment I grab the camera I lose track of everything else" - this kind of attention span problems sound like the guy shouldn't be diving with a camera in the first place, but some people feel they're better off diving solo to not bother other divers.
Rephrased: The original question was if solo diving was ever OK, you answered that many photographers do it all the time. Unfortunately, in most cases it's just what happens, but it's not OK from safety viewpoint.
Some discussions about solo diving tend to follow this flawed logic, which is related to the camera argument:
1. Solo diving requires solid skills, lots of experience and proper equipment
2. I've solo dived and am still alive
3. Therefore I must have solid skills and lots of experience
For the original question: yes, solo diving can be done, with the right diver, gear and easy enough dive it can even be almost as safe as team diving. On some very special occasions it may even be safer than team diving, but then we're talking about dives that are very risky anyway (sumps etc). On the vast majority of dives, it will not be as safe as with a buddy - so a person considering solo diving would have to rationalize the extra risk somehow. I haven't found a compelling reason to dive solo yet, but then again I have lots of extremely competent buddies available who are ready to dive pretty much anytime. I can understand why some choose to dive solo, I have no problem with that, what I don't get is them claiming it's somehow safer.
//LN