Using the whole screen is one of those things that sounds more efficient in theory, but with large monitors there's a lot of scanning you have to do. With the menu I can scan top to bottom in an alphabetical way -- very quick to do. With tiles I have to scan in two dimensions, and there's no particular order to them so I can't just skim over any of them.
It's not the exact same thing, but it reminds me of the weird misadventure they made in Office 2000 where they would hide infrequently used menu items. It sounded good in theory, I mean, less menu items should mean less confusion right? But in practice the main response was "where the hell did that thing I was looking for go?" and "why does my menu keep changing?"
The users can dictate the ordering of apps and folders on the Metro Start Screen. Not only that, users can incorporate spatial organization to make apps even easier to scan and find quickly. You could argue that this is a better design. However, all designs have pros and cons and are not necessarily best for all use cases.
As to your second point, I think the question is how do you improve your UI without breaking previous mental models/behavior?
It's not the exact same thing, but it reminds me of the weird misadventure they made in Office 2000 where they would hide infrequently used menu items. It sounded good in theory, I mean, less menu items should mean less confusion right? But in practice the main response was "where the hell did that thing I was looking for go?" and "why does my menu keep changing?"