I kind of agree. This can work beautifully and transparently for users, and the Nokia maemo/meego devices were heading in this direction I think:

Chat/instant messaging is handled centrally by an application, extensible with plugins
Sharing also can be done mostly in the same fashion,

Sailfish is a step backwards in this regard, mostly because the APIs are not available for development from third party. It is probably a temporary problem, but I really hope it is fixed soon.

On the other hand, I understand why third party developers like the "app" echosystem: they get to be visible, show their brand and get some revenue back.

In an open source environment, the pressure to create silos is less, but still, cerating your own application tend to be cooler than working on infrastructure or plugins.

But users would benefit from common services more than from thousands of applications.

By the way, the enphasis on application marketplace is misplaced in general. there may be millions of apps, but end users are going to install sone dozens of applications at most. The trick is to have the right applications available, of course.