I use a bar end mirror. Rode without one for a couple months, thought about getting one as I saw a few others with them, and then got into a potentially dangerous situation because I couldn't look back* and ordered a mirror when I got home. Love it. I could ride without one, but would not choose to. Speaking as a relatively old guy riding on the road.
*Road with no shoulder, guard rail 12" off the fog line, needing to make a left turn, and hearing an apparently large vehicle coming up behind me. I couldn't tell how far it was behind me, and didn't feel comfortable turning to look because of the guard rail. I ended up stopping, which even that felt dangerous because I couldn't tell if the oncoming vehicle saw me.