1. Do not take this statement for granted: "When your page is simple, yes, you don't need that much interactivity. But the market will demand more and more functionality on that page." Simple, fast apps that don't overdose on interactivity seem like a better bet. The vast majority of problems being solved with software do not require the bells and whistles of Facebook & Gmail.

2. A huge aspect of choosing Rails or client-side MVC is productivity. If you're going to build a web app in Ruby, you're going to have to have somebody on your team that is good at Ruby. By layering a client-side MVC on top of that, you're now requiring javascript expertise. Your life is now twice as difficult.

3. It's amazing how powerful DHH's approach is. Just look at the new Basecamp. Does any part of that app feel slow, clumsy, and non-interactive? Absolutely not. I imagine myself being opening up the Basecamp codebase and "getting it" within a day. If you understand your constraints well, you can deliver amazing software with an order of magnitude less complexity.