We like ponchos because they can be worn over anything. They are light, so we don't get overheated. They will cover a sweatshirt if it's chilly outside. We like the versatility of the ponchos, and we always use the Disney ones. Our first bunch lasted several trips, basically until the kids outgrew the child-sized ones. When they graduated to adult-sized ponchos, we all upgraded to new ones.
If we are travelling in the summer, sometimes we just like getting wet to cool off and leave the ponchos in the backpack, but we always carry them just in case.