I've always fit myself to my bikes and apparently it works well enough. However I'm not against going to a fitter just to see if there is anything they suggest that might help. Particularly if one is having issues they can't figure out. Some cyclist are just poor at realizing what's the real cause of the body issues they are having and what if anything on the bike is causing that.
Of course that's going to cost big bucks. But it's useful knowledge even if all they do is confirm you are doing it right already. Though I do have to say that I don't agree with the way some fitters go about fitting. But that's another story much like all the different diet fads.