I'm not here to doubt Jeff Berlin, or anyone. There are many ways to analyze how to be a successful musician. But then, what is success? That in itself warrants a separate discussion. I don't want to start it here and now, though.
But, let me say this. I understand how being able to read and understand theory can help anyone succeed (I have a degree in theory myself which has helped me at times). However, there are so many musicians out there playing who can't read and who know little, if any, theory. Right now, I am working with three bands in which reading has no bearing. None of the other guys, except one, in all of them can read. It is also a problem for me sometimes in that I can't relate something due to them not understanding theory. BUT...I am being successful at having plenty if gigs, and so are they. So, there is apparently more than one context in which to consider what success means. Am I wrong? I think to those who only want to play for recreation, as in a weekend band, having just a few, or even one or two gigs a month would be considered success. Many non-reading folks are accomplishing that. But, it depends on the individual and what it is that they WANT. Each person sees it differently. Surely there are more weekend warriors who do this, and who cannot read or explain theory, and who also feel satisfied---maybe not all, but many. Granted, pro positions will require those skills more. And even many other part-time gigs. I have played pro and semi-pro. Some of it was reading gigs, some not.
I suppose I could've said it all better by just saying that success is relative to who you're talking to. It's a wonder any teacher can figure out what to say. But, in general, I agree that any musician will be enlightened by learning theory, and that being able to read opens the doors for other gig opportunities. I've certainly played reading gigs, too.
Tip o' the hat to Jeff Berlin, as well as to others for their opinions.