Generally speaking demanding data form your users before they get to experience/use your app is bad design. Its understandable to a degree for twitter ,tumblr, facebook, etc, because their services transcend mobile apps, and include a website that your account makes uniquely yours.

If all you are is an app, and the first thing I need to do is give you my email, to use the app (or register an account), I'm uninstalling it.

:.:.:

Dare I ask why you want their email?