While I am always open to, well, opening things up, I came from the era of PocketPCs, where you could isntall anything you want and several app stores exsited. And while that sounds good for the user, it was a PITA. Different apps had different licensing rules - serial numbers, activations servers, differing activation install counts. A real mess. Plus, you never knew the whole breadth of what was available because app sources were all over the place.
The Apple app store keeps it simple. All he apps in one place, one licensing model, 5 device installs, nice and simple. And a relatively stable model for security. You could jsut limit yourself to the Apple app store to keep this simplicity, but I worry what changes will come to the app store if Apple is forced to open things up.