Thinking of it as a first aid item is a great idea, since I am sure many of us carry rain jackets because “that’s something we should bring.” Here in the mid Atlantic, rain on a summer backpacking trip is usually greeted with celebration and the removal of as many clothes as possible. It seems that there are many items that I carry that I would prefer not get wet before myself (at least in the summer), so I have something for the pack.
If we pack clothing which is designed to dry quickly, then perhaps the “need” of a rain jacket is less important – especially since there are usually other things in our pack which can keep us dry in a pinch (tent fly, tarp, trash bag, etc.. )
Granted, this assumes that one has an assurance that they have anticipated all the possibilities in the five day weather outlook. But if there is no risk of hypothermia, then it’s really only a question of comfort. Choose what is comfortable for you, knowing that one person may prefer a windshirt because it is more comfortable to wear than a rainjacket for most of the trip, and is typically lighter. The other person is fine with the rainjacket only and no windshirt.