I think the manufacturer's footprints are just a time-and-trouble saver, and of course they add some profit margin.

Tyvek doesn't seem it has any advantages. It's fairly puncture-resistant, but not really waterproof. I tried it once, and it seemed to pick up lots of dirt and moisture, when compared to polyethylene sheeting.

In Beyond Backpacking, Ray Jardine says, "I think the interest in this material is based more on the name, which has sort of a fresh, exotic appeal to those unfamiliar with it."

2- or 3-mil poly sheeting seems the best. I like it to be big enough to use as a separate ground sheet...not sure if the Lawn & Leaf bag is big enough.