I've been playing guitar for over 16 years, and for the first 5 years or so, was mostly self taught. After that, I decided to take lessons and the first thing that my teacher pointed out is that I was doing everything wrong.. Yes I knew my chords, and some scales. Yes I knew what I-IV-V meant. But what I didn't know was proper technique. I had developed bad habits over the course of my first 5 years of self instruction, and those bad habits were keeping me from advancing.
The biggest advantage of private lessons is that they can help you avoid bad habits and as a result, become a better player faster!