Because nonblocking is the secret ingredient in the async sauce, of course. With it, you go fast. Without it, you go slow.

Without all that callback spaghetti, how are you supposed to scale to roflmillions of users?

We really didn't make the choice to prepare for millions of users. We like having complete control over our code, and nodejs+express gives us that. I'm sure we make some mistakes Rails would correct for us and we take a bit more time on development, but we also learn a lot.

That's a tradeoff, and overall we're happy with it.