Or a combination of that and a way to run pre-orders, depending on the project. I think that KS usually encourages a fixed goal, even if that goal isn't going out to all donators, rather than serving as a tip jar. (Like, for instance, 'fund me to take my band on a European tour' or even 'fund me to work part time in 2014 so I can update my webcomic daily'.)

But I think the artist should be upfront about how much is just 'you donate money, and get things' and how much is 'you donate money and I continue to do art, that may or may not be available for free or be donor-exclusive', or the combination of the two*. (I'm generally in favor of transparent accounting, especially so that future Kickstarter projects know how to budget.)

* I know Rick Burlew ran a KS to get his comic re-printed, but one of the stretch goals was 'update the free webcomic daily for a week X times this year'. Given (IIRC) he is a freelance writer, in one sense that is paying him to work on the comic rather than other paying projects, rather than have the comic be a free thing he can do when his paid work is done. He's also been upfront when things like an injured hand meant that he had to delay any updates because it didn't matter how much he was paid, he can't draw when injured.