For the longest time I had ponchos in my luggage. I never took them out, we never wore them, and eventually I asked myself "why do I bother?" I actually kind of like rain, especially if it's hot outside, and I'd just rather get wet in between buildings than bother with carrying ponchos or umbrellas with us (and ditto others who have said that ponchos feel gross when it's hot and humid outside.)
I say this, however, with the caveat that I don't think I have ever had a total washout day at WDW. An hour or two of rain here and there, yes, but nothing where it was pouring all day, so my answer would probably be different in that case (I'd probably just cave and buy an umbrella if it was that bad.)