I have one, and don’t carry it during summer in Ohio, but in spring and fall I find the same advantages aimless does.

You’re right, a rain jacket can double as a windbreaker (but not vice-versa.) However, when you wear the rain jacket under a pack, the pack’s belt and straps will abrade the DWR coating and, eventually, the WPB fabric. I prefer to wear out a cheaper windbreaker, and think of it as expendable.