Though I don't have the capability to use an e-boarding pass, even if I did, I would stick with the paper one, partly for the reasons you stated. But also partly because I need the boarding pass to access the NW lounge (AmEx Platinum card). And what happens if/when NW doesn't credit miles? Do you send in your handheld device in order to get credit since they want a copy of your boarding pass?!
Though I enjoy the convenience of technology, this option is mostly designed for computer geeks to get their fix
.
Apparently the NW World Clubs also have scanners to validate your EBP. The issue of crediting miles I doubt will be a problem as they obviously cannot insist on a seeing a copy of your EBP. But I used the EBP out of sheer curiosity to see how it would work, and yes, because I am a technogeek.