Re: Scales & Modes
He didn't say it is vital. He said there's no reason not to learn it.
Music is an art, yes. But masters of any art had a basis in years of hard work. I don't think Da Vinci just decided to pick up a paintbrush one day and paint the Mona Lisa. Many of us have a little natural talent but it's barely worth a damn if you don't know how to exploit it. Technique is one thing we can work on, but without theory all you've got are some hand movements.
Unless you have a prodigal ability to play perfectly and communicate exactly what you want by ear, theory is extremely important. And to say otherwise would be like saying you only need three fingers to play guitar. Sure, you'll get by, but given the choice, wouldn't you rather have four?
Furthermore, an understanding of music theory is not only important for understanding your music and instrument, it is also a way of communicating with other musicians. If I said to you, right, let's have a jam in E mixolydian. Would you be able to do it? Or would you just be standing there like a lemon? And how else could I explain this to you? Short of playing each note individually until you remember them.
Back to the original topic, I recommend "Improvisation Made Easier" by Frank Gambale - everything is broken down in detail and applied to the instrument with examples and jam tracks.