Because normally, if you do autodidactic learning, you will make many mistakes that will hinder you to reach a certain level. And it is very difficult to get rid of those "bad habits" once you have them.
I used to make the same mistake. I learned to play the guit out of books. Then, after 3 years i decided to take lessons (i was very lucky getting one helluva teacher - a dude who studied music in vienna and learned from the famous Karl Scheit) - and he pointed out all the lil mistakes i made. I always tried to play as fast as i could - but then he said "You are trying to build the 7th floor without having finished the cellar".
Good basics are VITAL and no book can teach you as good as a skilled teacher can. :)
Cheers
-=[Azrael]=-
[FONT=Times New Roman]Holiness is in right action and courage on behalf of those who cannot defend themselves. What you decide to do every day makes you a good person... or not.[/FONT][br][br]