I wear mine when the temps are low (30's) or if it is raining and colder than about 60. You ought to bring them with you just in case (remembering how grateful you are that you only need to carry unnecessary rain pants 100 miles instead of 600k... ). If you do need them, you'll be happy you have them. (Put them in one of those giant ziplocks!)

I am bringing my rain pants and showers pass jacket this weekend. 60% chance of t-storms tomorrow and 30% for Sunday. I figure it may also just be cold Saturday night/Sunday morning so I may be glad to have them (along with my 25 pounds of cliff blocks - what's an extra few ounces for a pair of pants!)