So they can stand out and be better? Really, why do you pick one app over the other at the end of the day? I'd like to think it's because they either do it better than the other app or they do more than the other app. If these free built-in apps stay this way until SP1 or Win9 or w/e doesn't that just leave it open for 3rd party developers to fill in the gaps like they always have?
I also think there is incentive to have a metro version of a desktop app, even a complex one like the stuff adobe makes. At the end of the day it's not the full photoshop client from what I remember hearing so having a "simpler" or "lite" version of beefy desktop apps can only be a good thing. Say a user gets good at the lite version of the app, if they want to really dive into it then offer up the advanced/pro desktop version and you've basically got them hooked.
Are you honestly going to tell me that MetroIE or Music is better? The rest is only confirming my point.
Again, take away the crutch of the desktop version and tell me if that is still acceptable. The point is, until MS shows us if and how complex apps can exist in Metro, there is the same assumption you made, that the Metro Photoshop will be the basic 'essentials' version and Pros will use the desktop. You are still describing a fundamental two app system, so that is an argument for two different OSs - which is not what anyone that is 'pro' Metro should be happy about.