There are many applications in Ubuntu which perform the same function, for instance Liferea, Firefox's Live Bookmarks, Akregator and others are news feed readers (RSS, Atom, etc) but they come with different default subscriptions. Different internet radio programs also come with different predefined streams.
I think the "extra feature" quality of these subscriptions should be merged whenever it can apply to all such tools (so if Liferea comes with Slashdot, Groklaw, etc. then why doesn't Akregator?). This could also apply to "Get More Wallpapers/Emoticons/Sounds/etc." type functionality.
Basically, anything that doesn't download large unrequested things (ie. downloading some default podcasts would be bad since they're big files) should be added to all capable apps.
PS: A very long-term way of implementing such a feature would be through unified backends like Akonadi for news feeds, DXS for application addons (like wallpapers), etc.