I never have found a need for a rain jacket, as I sweat through my jersey very quickly, but I could see how if you're not hammering, you'd like one.
I've found breathable windproof jackets to be the best very helpful on cold days. However, the Gore Windstopper jackets are usually good enough to accomodate that without needing to stash it. On hotter days, just go without the rain jacket.