I've got a pair of waterproof trousers that I bought years since in a sale somewhere, they're spray way. I have been running and walking for ten years and never needed to put them on. The only time I ever used them was at a music festival when I saw the Who and it was chucking it down.
It is something that when you do need them you'll want some that are waterproof and windproof you probably won't care about their breathability because you'll just want to keep the wind off you.
I hate even walking in waterproof leggings never mind running, as long as your bottom half is thin and quick drying you're okay. I didn't put mine on in the OMM last year and I'm sure you heard how bad the weather was there !