There is no incentive... until there is. The default camera app doesn't zoom. The default music player won't create playlists (edit -- sorry, it does; what I meant is that there is no queuing to the now-playing like in old iPods). The default todo app won't do cloud sync (well, it does now, but it wasn't the case before). And so on -- as soon as your usage of a particular app increases over a certain threshold, you'll encounter a corner case and head for the appstore to get a better app. So a photographer will have five picture-related apps and never touch the default one, while using the default mail client; a lifehacker will have fifteen hyper-specialized ToDo apps and never touch the default one, while using the default camera app.