iOS Outlook Web App : Microsoft Flat Metro UI

So when you download the new Office Web App for iOS, the first thing that hits you in the face is the UI! By all accounts it's similar to the Windows Phone and Windows app.

I first noticed this in the Facebook app, which is consistent across iOS and Android. Obviously Facebook considers it's brand identity more important than the different platforms it inhabits.

What do people think of this? Should apps for each platform conform to the design guidelines of that platform, or should they be consistent with the branding and/or design guidelines of the company that produced the app?