> Over time, any poncho is going to develop pin holes.
Do you mean groundsheet? I can't imagine how a poncho would get pinholes. Most of the time, I'd be camping on alpine tundra - so soft, lumpy grass. As such, I'd imagine that I'll be easily able to avoid pinholes most of the time, no?
> do you really need a groundsheet? A water proof foam pad should suffice.
If there's moisture on the ground, I can't imagine how I'd be able to completely avoid getting my bag, especially when used as a quilt, at least a little bit wet, with only a 20" wide pad underneath? Often times, our afternoons start with a thunderstorm in the mountains. The ground may or may not have dried off by the time I set up camp. The soft (and lumpy) tundra grass is often a little damp.