I wonder if we'd ever have heard the name Eddie Van Halen if he'd been a fat bald guy. Or AC/DC wearing suits and ties with short hair and glasses.
In this business, if you want to make it HUGE, then 50% of your appeal is your image. Nobody would give Britney Spears a second glance if she looked like my grandmother. And they wouldn't listen to her cause she sounds like my grandmother.
And I'm still willing to bet that the only reason Satriani shaves his head is cause he's got a receeding hair line and doesn't want anybody to know it.