Stay dry in the rain. That's the general idea of waterproof coats. I don't know why they are so expensive and difficult to get right. I've tried enough of them, as you can read on my blog latest "All the coats I've ever used"
Coz this is on the Audaxing blog there is some bias to the slightly different needs of the audaxer: the coat has to hold up for hours and hours of Wales and actually keep the rider dry.
Whereas on short trips if you get wet and stay warm it's not so bad.
Also, it's not like proper touring (or commuting) where the durability of the coat probably wins over weight.