I think his point is if "You are only designing web apps that your will own and operate or are in a family of web apps that will have a similar look" then it is ok to skip photo shop
I agree. I think, like it's been said, that 37Signals is a special case. I believe in the field we all work in, it is wise to assume a graphic mockup as a step in the progress and then carefully decide whether or not it's a neutral step based on the history of projects you've completed. In other words, there is no bottom line.
I don't think 37signals is necessarily a special case - there are plenty of customer oriented websites out there that have a look and feel, and don't require client buy-in.