Ted, I agree with you that having a major player enter the scene really helps the credibility factor, but I think Harley brings even more to the game than that. You say that the Volt brought the EV to the American public's perceptions, but to me, Tesla's pushed it much farther. Since Tesla introduced the Roadster, and started getting some press for it, EVs graduated (again, in peoples' minds) from being a glorified golf cart for ex-hippies and super-frugal types, to being exciting. It was the first really BADASS EV, and I don't think you can discount that factor. People may look at the Volt, or the Leaf, or the Prius as cars they might actually potentially own, but nobody drools over them. Tesla changed all that.
If Honda had built an electric proto and hauled it around giving out test rides, it would add a lot of cred to the electric motorcycle scene, but Harley made a bike that isn't just well-engineered, and reliable, and all the things that Honda does so well. Harley made it BADASS. You can ignore competent quite easily, but you'll never ignore badass.