Having worked at a place that sold web design packages in the past, I'm fairly convinced that I'll only work for design/development clients that can afford to pay hourly. That being said, this could be a good way for them to pick up some narrowly scoped work from their existing client base. Assuming that customers understand the deal, it could work. In my experience, they'll want to make changes, and the best way to handle that situation is by the hour.