The problem with extending another app is that you are now forced to have to update and modify that extension with virtually every update of the target app. Apple has a long history of late breaking, and often unnecessary, changes to their API's which already force you to virtually re-write apps with every OS patch or update, now imagine every time specific apps are patched having to rewrite your extension to work properly.
The idea is nice in theory, but Apple does not have a history of respect for developers with regards to stable API's. I can imagine that the interfaces to support App Extensions will change frequently for the next few versions until Apple settles the API down, so I won't rush to try and support this ecosystem until a few versions have gone by, unless you love rewriting code over and over again on someone else's release schedule.
The only other problem? Look at the disaster that browser extensions have created, my Dad has like 20 tool-bars and tons of extra buttons in his Firefox browser simply because of the rampant "extensibility" allowed. Hopefully Apple will reign in the proliferation of crapware that will be allowed to extend their apps.