We use funnels as a measurement of product quality all the time, especially in the context of understanding task completion rates and discovering areas for UX/interaction improvement.

The "funnels" that Ben seems to talk about are more about sales funnels where you keep getting pestered once you're a lead but in the context of most e-commerce sites, funnels are a great way to know if there's something about the site that's not working for people.