I close the lid to sleep it whenever I'm not using it. It never gets shut down except for, say, RAM upgrades. Uptime is usually measured in the months, I only restart for software updates or if it crashes.
I wonder if it's simply related to habits you pick up. All of my friends who own MacBooks who used to own PC laptops, always shut it right down when they're done, and start it up when they need it. I keep telling them they can simply close the lid, but it's like they don't believe me, or more likely, they don't trust that it actually works.