We have two happy, healthy dogs and we rarely take our dog for a walk when it's raining. Exercise is cumulative. On the week days when it's not raining, our dogs get a walk (or run) and either fetch in the backyard and/or dog park. On the weekends, it's usually a combination of all three and/or a hike. They get a lot of exercise, so I don't worry if there's a day or two here and there where they don't get much. On rainy days, if they seem antsy, we'll throw the ball up the stairs or let them wrestle in the basement (we don't usually allow them to wrestle in the house). When we only had one dog and he was still a puppy, we used to drop him off at the doggy daycare once in a while.
Few people can say that their dogs get as much exercise/attention as ours and I'm not concerned at all that I don't walk the dogs in the rain. We are an active family and we enjoy making the dogs a part of our activities. I highly doubt that all the sanctimonious posters who claim to be walking their dogs every day come hell or high water are actually doing that on a consistent basis. Or they believe that a leisurely walk or two per day is actually good exercise on it's own. Walks are great (for humans and dogs), but a dog should be panting for it to be exercise. Their dogs are often fat.
Of course, if you don't have a yard for your dog to do its business, that's a different story. Then I suppose you have to buy a good raincoat.