I usually bring one of these: http://gossamergear.com/sleeping/sitlight-sit-pad-group.html
I use it as a frame for my pack, a sit pad, a knee pad, a pillow, an extension of my sleeping pad, a stand pad on snow, a stove wind break, and probably a few other things. Our last trip involved igloo camping, and I spent 5-6 hours on my knees on this thing. It was a total life saver. I'm not against the idea of knee pads, but it's hard to see many multi-use options. But if they're light enough, that may not be an issue.