Personally, I go for a lightweight, breathable jacket - Marmot Mica - and pack something far more lightweight, waterproof, windproof and warm in my pack/bumbag: a black bin liner. Dr Mike Stroud - who knows more than most about surviving in difficult weather conditions - recommends bin liners over foil survival bags. I'd take two if running in potentially dangerous conditions as then you can get full body cover. The Mica is 'waterproof' to 20,000mm, which is pretty good, and is very breathable so perfect for wearing while working (remember, you generate about 2kw of heat when working hard, so a couple of layers and you shouldn't get too cold no matter how wet - if you are still cold, then you probably need some food down your neck). If you have to stop in inclement weather, get all your clothes on, bin liner over the top - job's a good 'un!
in reference to earlier comments about other lightweight so-called 'waterproofs'. I have a montane feather lite which is superb in winter as a windproof, but you're soaking in seconds in anything more than a light shower. there is always a pay off between breathability and water resistance, the key is finding what suits you best (and meeting race requirements - most fell/mountain events will require something fairly waterproof with taped seams) but for emergencies, I remain an advocate of the simple plastic bag!