Back to the OP, and to comment on various points raised - I make and use ponchos - they're my favourite foul weather gear. Waterproof. windshelter, pack cover, sit mat and groundsheet protector all in one. But they can be a pain to put on on your own!
That said the the standard commercial designs are too short (even for me) too narrow to keep your arms dry and have excess material around the legs.
My own ponchos are essentially a truncated diamond shape with an extra "back" section to give an even skirt length when worn over a pack. This shape also gives extra width for the arms and less material to flap about your knees. It was originaily designed to use as TN Laser Comp groundsheet (with the flap folded under), but the extra material also makes it fit a Nordisk Telemark with its asynmetric central hoop.
I've found it's best to attach the poncho to the rucksack before putting the pair on together - not the quickest of manoeuvers, but you can do it in the wind.
When worn, the central elastic tie-out loops that connect to the tent's main hoop are slipped over my middle finger to keep the "sleeves taut and arms covered.
The tent corner tie-outs are connected to hold the sides of the skirt hem together. Being elasticated they provide extra give when climbing stiles. So far I've never bothered with any sort of waistband.
Cheap plastic ponchos are very clingy in the wet. I currently use a medium weight woven polyester for its bramble resistance. It's not 100% waterproof, but the inside is never wetter than damp, it dries out almost instantaneously in any breeze and it does not cling at all.
In foul weather, a poncho provides you with your own microclimate so you can use a much lighter jacket - saving weight.
I also like brollies, but the windproof ones are far too heavy and the lightweight ones break! I'm still trying to find a design that has the right compromise on size, weight, durability and cost!