It probably isn't necessary in the strictest sense of the word, But I guess that really depends on what it is that you want to get out of it. But the most valua

It probably isn't necessary in the strictest sense of the word, But I guess that really depends on what it is that you want to get out of it. But the most valuable part of my PhD were the few years that I spent getting regular face time with subject matter experts, who walked me past the periphery of my field, to the point where I am comfortable enough publishing my own peer reviewed research. You could read the literature and try to get far enough up to speed to publish entirely yourself, but I guarantee that you will miss many subtleties if you don't connect with folks in the field. Enough that you are face a lot of hurdles trying to pass your work through peer review. The idea of doing that alone seems twice as daunting as doing a PhD. That said, I am neither teaching nor in academia (gov't at the moment). My retrospective might be a bit different if I was trying to elbow my way into tenure.