I think this is an interesting problem but frankly, I haven't found the ultimate solution yet. Rain isn't the only problem but also snow and sometimes, wind. One of my favorite places to go is on a trail that follows ridges, up and down for miles. The places with the best views (the chief reason for going) can sometimes be quite breezy and for a lengthy stop, a good windbreak is appreciated. As an aside, I spent one night at a good spot for a camp along that trail and it wasn't breezy at all but it was foggy. The trees dripped all night long.
At any rate, I've tried ponchos, zeltbahns, shelter halves, tarps and combinations of the three but they all fall short in one way or another. They're either too open and provide too little wind protection, or they're too heavy or just plain difficult to put up. I've never attempted any in a heavy rain. I've thought about other alternatives, including beach shelters (too heavy and too expensive), bigger tarps (promising) and small tents (also promising if they're easy to put up). The only problem I have with the tent I have now, which is an ancient Nu-lite, is that it isn't something you can set up in much of a hurry. But it is light and insect proof. Sometimes you want something to provide shade, too.
I do like ponchos myself and I find them almost ideal for rain but the one I have is too small to make a real shelter. I also think an umbrella is practical and I use one, too, if I remember to take one along, which I usually don't. The biggest problem, at any rate, is that most of the places I choose to stop along the trail for a long break do not seem to be the best places to make a shelter, usually because the land is steep and rocky, but we manage.