Regarding your switching point, I'd definitely switch (and, so would anyone who uses vi/emacs (maybe I should market to them)), but I realise I'm not my target market, and that your point applies to that very much indeed. This just means that I have to make every aspect of historious easy to get into, though (fully-functional demo, easy account creation, extensions for all browsers), as I'll never quit :P
On an unrelated note, I just took a look at moreofit, and I'm very surprised by how accurate it is, so props for that!
I don't see any reason to require a user to create an account to start using Historious. Just create a unique hash (user id) in the bookmarklet, and have that hash be sent along with any "historify" requests.
Perhaps once 30 days are nearing, or they are filling up their free account, you could remind them that they should upgrade to pro, or at at least create an account.
Hope that helps.