I purchased the 30mm wide XMIplay carbon rims, in my opinion that width is sufficient for the Cross Country riding I do. I could have gone with a more narrow rim and save a little weight, or I could have gone wider and added weight and strength. I tend to believe the rim width is a bit of a personal choice as to what you think you need.
Also, for the longest time, XC wheels were on the order of 24mm wide so the increased width is just giving people more choices, and there is nothing wrong with that.