Imagine if Apple started out as a company that distributes music and designs handheld devices before creating a computer. It's not hard to see a Mac OS that only lets you buy apps from an app store.
Without the risk of an insecure cellular network, greed would be the only motivating factor in keeping this a closed device.
That's probably true for the iPhone, but it doesn't explain the closed nature of the iPod Touch. The two devices share the same core OS, but it wouldn't be difficult to let iPod Touch users download apps outside of the app store. So if its not about protecting the cellular networks, and it's not a technological barrier, what's left? I guess you could argue it's about control or about enforcing a consistent (and "safe") user experience. I'm hoping they don't apply the same thinking to the Tablet.
What about the rumors that the Tablet will include nationwide 3G access, similar to the Kindle? I believe this is a necessary addition -- think about how useful the iPhone would be without Edge/3G access: not at all.
Maybe you wouldn't.
But I would. Because when I need mobile computing, I generally don't have a desk, or even a chair. Sometimes I'm actively walking around. And I don't need to use it for sessions of length or complexity that justify the overhead of the desktop computing model. And I don't need arbitrary pre-existing desktop apps.
I just need awesome core apps that I can get at in a couple seconds, use (literally) on the go and be able to get through a day without recharging or hassle (lock-ups). It doesn't have to replace my laptop. Just my dead-tree notebooks.
The iPhone has come far, far closer to working for me than conventional laptops, netbooks, or windows tablets. It just doesn't have quite enough screen real-estate for some tasks and the ergonomics aren't right for others.