I have a 13 oz Sea to Summit Poncho Tarp which sets up beautifully as a shelter (to be replaced with a 7 oz Golite Poncho Tarp as soon as they're back in stock, assuming the price doesn't keep going up).
However, the problem exists that once the poncho shelter is up, I can't go outside. That's a real pain--it would be impossible to hang a bear bag, go to the bathroom, get water, or anything after the tarp is up.
So I ordered four 1-2 oz cheapo ponchos that are little more than a garbage bag for $5.32 from amazon. During the day, these would be way too flimsy and non-durable to hike with, but for quick trips out of the shelter just to stay dry, they ought to do the trick. If they only last a couple weeks camping, that's fine--at a cost of $1.32 apiece, it's worth the savings (although I hate the thought of all that trash!)
Total weight of the go-lite poncho tarp system, including string, aluminum stakes and GG polycryo groundcloth, would be around 12 oz (or less than 20 oz with my current poncho). Total cost of the system, if starting from scratch, would be about $70-$80.
Correct me if I'm wrong, but isn't 12 oz for raingear and complete shelter system less weight than some of the much more expensive options?