Yes - I have an Integral dynamics Silponcho - which pretty much becomes a 5x8 tarp.

You can stay nice and dry under it, even in very foul weather, but I have found that doing so takes some work - you have to be careful about site selection, and with a 5x8, if it's really bad, you have to pitch it pretty low, i.e. low enough that I can't sit up in it. None of this is a dealbreaker, it's just an issue of comfort. if you're ok getting dressed and ready lying down in a thunderstorm, ok.

Realisticly, my silponcho I think weighs about 250
grams. The big weight saving is that it *is* multi use.

but to put it in perspective, my SMD lunar solo tent weighs about 680 grams, with room to sit up, sit out a tstorm, and bug netting.

The other possibility is a bigger tarp. it is *MUCH* easier to stay dry under a larger tarp in foul weather. I have an integral designs silwing that isn't much heavier than the poncho, but has a lot more coverage. an 8x10 silnylon tarp shouldn't weigh much more than 350-400 grams and will be MUCH easier to rig for foul weather than a 5x8..

So really, it's about plain old comfort.

But yes, for absolute lightweigh, IMO. a dual use poncho is about as light as you can possibly go. I did it for "fun" (and actually it was) - but doing so was instructive enough that I was willing to (myself) carry a few hundred more grams for a bit more comfort and versatility of site selection in bad weather.