I do think we need paper -- a "hard copy" -- at some level, whether for the ballot itself or just a verifiable audit trail. I think 100% electronic voting would be fine so long as there's an audit trail that matches the (electronically) talkied results perfectly. I don't think dealing with millions of paper ballots would be any more advantageous than that -- you'd have to distribute and collect them (although that would happen at the centralized polling places), a few people would have to hand tally them (to ensure everyone's numbers match up), and then securely store them (via very strict "physical security" procedures/protocols) until, at some later point, they can be properly destroyed.
Logistically, electronic voting with a paper audit trail would be much easier, would require fewer people (i.e., election workers -- who are already in short supply), would give us results much quicker, and would be easier to verify/authenticate later (in the event of disputes).