I've never been one who wants to get early access for beta builds of games, and I don't think that would change for Ukulele. But it's weird. I'm extremely pumped for the game, and if I were to be offered a chance the play the game ahead of time, you bet your ass I'm jumping on that opportunity. However, that's only if it was just a short demo or something.
Extensively playing a beta would just ruin the experience for me. For one, I'd be playing a relatively significant portion of the game before I get the full experience, thus hampering it when that finally happens. Also, it's incomplete. Granted, demos are as well, but they're prepared specifically. Playing a beta would just get me to associating the game with that initial experience of playing an incomplete product.
That said, I understand that playing betas before the game is released is a lot of people's cup of tea, so I wouldn't mind if Playtonic gave those whose boats are floated by that kind of thing the chance as a Kickstarter reward or something.