Third parties can add similar functionality to their live tiles too, and a lot of apps do that. For example, the package tracker app can update the live tile with the latest location of the package, the twitter app can update with the latest tweets or tweet count etc. That is the whole point of the 'glance and go' mantra. When you're busy doing other stuff, you can just glance at the homescreen to see if there's something important or not. A bunch of icons like every other phone UI designed by Xerox PARC in the early 1980s doesn't do that.
I "get" live tiles and I kind of like the concept. I'm saying that their size has disadvantages, though.
Three "pages" of apps on an iPhone (which is pretty average) hold 52 apps (16 x 3, plus the 4 in the "dock") that you can quickly navigate between. From what I've seen, anything more than 20 apps on WP7 seems to be more trouble than it's worth. At that point, you're exhausting the limit of "glance and go". Anything beyond that you relegate to the drawer of apps that few people actively use.
Those 20 apps are better reserved for the ones that do live updates, although I agree the app drawer can get a little crowded(there's alphabetical search in Mango to mitigate that a little).
But wasn't there a study showing how many apps people use on average? I believe it was far less than 20. Someone made a concept app to add folders to the UI(need to be dev unlocked or jailbroken to use it) http://www.engadget.com/2011/10/25/want-folders-on-your-wind...
