If you're a violinist, cellist, hornist etc. it's very helpful if you want to work in an orchestra,
but there's not many orchestras hiring guitarists these days, nor has there been any in the past.
For the most part I don't think you do it just for the diploma, but for the opportunity to really
grow with your instrument. Let's face it, 3 years of full time guitar-studies will
probably teach you one or two new things.
In classical guitar at least, your chances of success without college education are very slim.