I'm not a purist. I use whatever tools make sense for the context I'm in, and when it comes to organizing projects, I use whatever life cycles–in whatever combination–make sense to me. In response to a mailing list query, here are ways I've used life cycles for a few projects.
Let's assume you're collaborating with another organization. You would like to define an architecture sooner rather than later. You're nervous about an architecture that emerges from implementing some features–you want a little more planning than that.
So you decide to prototype the architecture for a little while in the project (an iterative life cycle), and then move into implementing by feature (incremental life cycle). I've used this combination life cycle with and without timeboxes.
Is this a perfect life cycle? Nope. But it beats the uncertainty of a waterfall.