He went to LA to be a musician(then fell into acting)as self-satirized in this video…
I've seen him sit in with Ian Hunter, and with Beck years ago. He was not exceptional, but he was competent. Beck does whatever the hell he wants, I don't think he "needs" Depp, any more than Depp needs Beck. I listened to an interview with them on Little Steven's Sirius Radio channel, they have a genuine rapport, and there was deference to Beck on Johnny's part. I can think of worse musical 'posing' on the part of other actors…Bruce Willis, Steven Segal, even Belushi/Aykroyd. As for the HV, it's just a side thing for fun, a Busman's Holliday, a chance to play favorite covers. If Depp was so miserable, why would pros like Cooper and Perry put up with him? They have their own following. It keeps people entertained, and it keeps a friend of mine, Chris(the bass player), employed.