I keep one of these in my vest at all times:

http://www.cabelas.com/cabelas/en/te...lite&noImage=0

It's GoreTex, so I know the quality is there, it's their PacLite series, so it packs very compactly and it's light, which I appreciate especially during the warmer times of year when a shower is likely to pop up. The way I've always looked at it, I don't want my rain jacket to add insulation, just to keep me dry. Unless I've screwed up, I should have the right amount of clothing for the temps I'm going to encounter that day before I put my jacket on, regardless of whether we're talking winter or summer. Especially on a warm day though, I don't want a heavy rain jacket adding to how hot I probably feel already, even it is breathable.