The idea is that there is already a lot of weight on the back half of the bike, so placing a good bit up front will help balance the overall weight load.
Too much weight in the back(ie all of it) makes for an unsteady ride when going downhill.
With all that said, I've done multi-day trips with about 40# all on the rear rack and never had an issue or even thought about the weight. I was on flat to gently rolling terrain though.