it's weird. XC is one of the most instense endurance biased sports but you look at the muscle tone of the top guys and their upper bodies stay realtively developed. from the prevalence of double pole work. (now now...don't be nasty)..
I've tried it a little over the years as i switched to strength biased sports and it's very soothing. takes advantage of a strong psoterior chain and hips but man I gassed quickly. Skated skied a moderate difficulty 5K course last christmas that just about killed my fat ass.