Yes - if you could get access to "a bigger social graph" without risking the owners of those bigger graphs shutting down or limiting your api keys, competing with you directly after you've proved/developed a market, or any of the other "dirty tricks" Twitter/FaceBook et al. have pulled on developers.
(Note: app.net might pull those dirty tricks in the future, but at least they've got a business model that doesn't _require_ them too)
Twitter only allows you access to 100k users or so (tokens), and your business is at risk of being turned off.
or
App.net providers you access to 150k+ users growing user base. Plus an a free sign up, access to hosting for file storage, and permissive messaging.
(Note: app.net might pull those dirty tricks in the future, but at least they've got a business model that doesn't _require_ them too)