Meh to each his own I say. While I DO use boots in any competition b/c horses tend to overreact when stressed or excited, I am a firm believed in schooling w/o boots to allow the horse to build up strength and conditioning. Much like racehorses will benefit from conditioning over varying terrain, so will any other equine athlete. By not always using support, the soft tissue as well as the bones will remodel according to the presented stress, and go from there. While there are studies that show boots such as SMBs offer minimal support, there are also studies that show the opposite. My bigger concerns XC however is finding a boot that does not retain water, thus increasing the chance of a bowed tendon or other leg injury (check ligament, suspensory, etc.).
If you disagree, I suggest you put your horse's SMBs (or ANY boots for that matter) and jog around, then jog through knee-deep water, then continue to jog afterwards and get a feel for the weight of the boots on YOUR legs and see if it adds or decreases stress. As most XC courses have water obstacles of some sort (esp in the higher levels) it would seem that these aren't a good idea. I haven't competed at the higher levels myself, so can not speak from personal experience, but I CAN speak to the jogging through water exercise and it's certainly MUCH harder on the other side of the water ;)
Ulster boots are very popular XC b/c they do not retain water, but I'm not sure that they stil make them. I forgot but there are a few other brands as well - I'll see what I can find if anyone is interested.