You don't have to soak beet pulp. Some people do, as it used to be thought that if you didn't it would expand so much the horse would colic (that has been disproved, but some still believe that and swear by it, but I have never had a problem). If your horse has never had beet pulp, soaking it can make it palatable if they don't seem to like it. (it has a very coarse texture) In winter, as a nice measure I might soak it in some warm water. On the flip side, beet pulp will turn rancid quickly when soaked (it begins fermenting), so if your horse won't eat it, it has to be thrown out.
I use beet pulp if there isn't enough quality forage available. (I try to stay on a hay-only diet, but sometimes extra is needed when hay is hard to come by or of lesser quality than I'd like) In that case, I use primarily beet pulp, and add in a small amount of feed to give compensate for the nutrients they would be lacking.
I've only given rice bran to young horses that were underweight, as it has high fat and protein that help gain weight/muscle. Once they are getting where they need to be, then I haven't used it. I have known people that used it with horses that are in intense work, and are young and growing. I myself wouldn't recommend it as a every-day supplement unless you work your hard every day, as they don't need it and can cause more problems than it's worth. (Problems being; anxiousness, hyperactivity, etc- all that energy has to go somewhere!)