This is a great question
I was self-taught when I started drift diving in SE Florida about 10 years ago. Prior to that, I carried a surface inflated marker and never had to use it. Deploying a SMB was not taught in my OW or AOW class, but that was quite a while ago. I was required to deploy a SMB twice during my SDI Solo Diver course, once from the primary and once from the pony. By the time I took this class, I had been deploying a SMB periodically for 5 or 6 years. I use a spool rather than a reel.
Much of my drift diving is solo and I have my flag on a reel and no need to routinely deploy a marker. When I have others with me, I generally carry the flag also. As they say, the person with the flag is never lost

. I dive with some operators who put a DM in the water with a flag. As you are not required to stay and ascend with the DM, not infrequently, I send up my marker. On a few occasions, the line to a wreck has been "mistakenly" removed before I finished my dive and I made my ascent on my SMB. This is particularly important when there is brisk current as you can cover a lot of territory between the ascent and the safety stop. I used my SMB several times in the Red Sea and was prepared to use it in Cocos, but never had to.
Deploying a SMB is an important, perhaps even a critical skill, if one dives in situations where it would be needed. Many divers probably never dive in those situations. Deploying a SMB should probably be taught in AOW with a couple of observed deployments, perhaps once on the deep dive and once on the navigation dive. Divers who need the skill will continue to use it, others never will, but will at least know about it.