If you're horse has a spot in the pasture where it can get out of the rain of its own volition, you probably don't need anything. For the most part rainsheets are probably a human convenience thing..ie, I can go out to the barn and right away and I can put my saddle on a nice dry back.
If there is no shelter in the pasture, I do like to use a rainsheet as the horse has no other way to get dry. But like SueNH says, you're doing more harm then good if you throw a wet cold rainsheet on your beast and then toss him out into the rain. So having a couple that you can rotate during the wettest times would be the best.