It depends on what kind of results your looking for, I'd say.
I have clients with whom I've worked out this per-hour approach: My client have a list of tasks that they keep filled. I take out time and work on the tasks with the highest priority. We agree on beforehand how many hours, roughly, I should work every week. This means, for them, the cost is pretty constant, but the results are varying. Usually we also agree I can deviate and work on smaller impulsive tasks, ie follow my "spur of the moment"-creativity. Then once a week we meet up and discuss my work. (That regular meeting is important.)
The benefit of this is I can work whenever I want. This may sound sloppy, but what happends is you end up working when you have ideas and a feel inspired. If you want "innovate" creative results, I think a setup similar to this is almost required.
The down-side is there can be weeks where nothing significant happends. If results are needed on a certain date, I don't think this works that well. It also requires you have a healthy business relationship and can trust eachother. I wouldn't do this from day 1. I would say it also requires that the creator is passionate about the project.
Then I have clients with "normal" projects where the desired results are more or less known and there's a set time frame. There's also usually a known budget. Here I think the only viably approach is agreeing on a set price for a given amount of results of a certain quality. The benefit is the chance of getting those results at the given time is much higher, and that means the client can plan the project. The down-side is you seem to slip into this "daily-grind" mode, as a creator. You run more on auto-pilot and mostly stick to methods you already know. Since you know what you need to deliver, you deliver that, and that's it. Then of course, that also means you usually get more done in less time.
I have to say I like both approaches. They compliment eachother and mixing them up keeps me evolving new skills as well as improving my already existing ones. My clients seem to like both as well: One delivers out-of-the-leftfield ideas and the other gets things done.
I'm not sure I quite answered your question. But at least I think these two methods are related to being paid per-hour vs. per-project, somehow, I hope.