Rain pants are not always necessary, but in colder climates I would not go without them. There are also some interesting varieties of pants that can be used.
I really like rain chaps in combination with an umbrella for rainy weather. Chaps are difficult to find on the market but easy to sew. These are homemade.
For really windy and rainy weather you need to protect your lower torso (a k a covering your ass) and a pair of homemade shorts in waterproof breathable fabric are excellent as complement for the chaps. This setup can also be made from two pairs of commercially available rain pants. Maybe you have one pair with holes somewhere. Or you can buy two pairs of cheapies, like Driducks (see under Rain jacket).
My current rain pants Marmot Essence 2010. Weight 210 grams. I do not need the leg zips since I wear light shoes that slip easily on and off. But the weight penalty is very small and the price is attractive.
Comments and current insights: I find that chaps in combination with an umbrella handles about 80 percent of the rain I hike in very well; especially in the forest, where the wind normally does not constitute a problem. But you need to be able to cover your lower torso when needed. I have walked against hard wind and rain and soaked an unprotected groin, after which the water ran down my legs and soaked my legs completely. Not something I want to repeat. See also the chapters on Umbrella and Rain in general.