I have always considered it a badge of honor that I coach myself and have had moderate success at doing so. Most of the guys I run with have the same mindset. Lately, as I enter my mid-30's and seek loftier goals, I've begun to reconsider this position.
My normal training formula of 75mpw, weekend long run, and mid-week track session have done wonders... I've gone from 16:20/1:14/2:45 to 15:25/1:09/2:33 and I wonder how much further I could drop given the right guidance. I've also read a lot of marathon success stories on these boards that included something like, "and then I connected with a coach and [positive outcome x,y,z]."
In other words, I have always had the opinion that non-professional runners don't need coaching. Ironically, I seem to be reconsidering this stance as I get slightly faster.