I'm only doing lifecycle emails on some of my products. Some of them were built way before the current lifecycle email craze, and I haven't gotten around to adding them on the others yet. On those older products, I do send emails asking them to come back and enter credit card details.

Great point, though! My larger point, I guess is about leveraging something that only more interested users will go through (credit card entry in this case) to let them filter themselves out early.