This is exactly why I use the 79-cent "disposable" ponchos! They are small enough (in their original packaging) to tuck into a pocket or fanny pack, and you don't have to try to fold them back down again like the WDW ponchos. Toss them after you use them - they're so inexpensive, you could buy 7 or 8 for the money you spend on a WDW poncho.
(That said, though, I think I might invest in a WDW poncho if it was truly going to rain all day long. The time of year we go, we might get a shower or two that lasts an hour the whole week we're there. Last trip, it only rained once. If it was going to rain all day, especially for two or three days, I'd invest in a heavier-weight WDW poncho.)