I thoroughly recommend waterproof trousers! In fact you guys have missed out on a pair of Gore ones I was selling on the Classifieds yesterday.
They have been used in all sorts of horrible conditions from winter riding on a 2,000 ft Welsh mountain to the monsoon conditions at last month's Swinley Forest Forum Ride. Have always been warm and dry. Of course I wear them with SealSkinz merino socks for added cosiness.
I have just purchased a girlie pair of Gore waterproof trousers so they will actually fit me this time!
Being full-length obviously means that you have more chance of keeping your legs and feet dry plus it's easier to strip off at the end cos your legs are clean.
Yes, they are a silly amount of money but Gore kit always works for me, lasts for years and does what it's supposed to do. Plus, of course, I'm worth it 🙄