The drawback of this method would be a lack of compatibility with Windows XP icons. Actually the PNG-compressed Vista icon file format is compatible with XP because it includes the small XP image formats (48x48, 32x32, 16x16). To read more about Vista icons read our article.
However, it would be a good idea to support XAML vector image instead of (or as an alternative to) PNG in the final Vista icon file format. Microsoft developers, do you hear me? 
However, IMO, a simple vector image is not the universal solution. We think that even if a 100% vector icon format is implemented in the future (when XP compatibility won't be necessary anymore), it would be necessary to include at least 2 versions of the icon image: A detailed one for large views (thumbnail, preview, desktop display...) and a simplified one for small views (taskbar, app window title bar...). Technically, it is possible to display a very detailed vector image in the taskbar (say 1/4 inch size) but it would be difficult for the user identify it. This is a basic notion of icon creation: small images must be simplified, details removed.
I'd like feedback on this subject.
Regards,
Marc
