I used to feed 18% to my show rabbits. I simply thought it was better at the time. In the last year, though, when comparing the feeds, I can't see any quantitative advantage. I suspect that in most cases the extra protein is simply wasted. I can't be sure of this as I feed a little "calf manna" to my lactating does, and it may be that this makes up for the difference in a situation where the extra protein matters.
I noticed no extra weight gain in growing out kits with the 18%. I feed greens when they are readily available, but the amount is so small as to be considered more of a treat and a supplier of extra vitamins and minerals than having any real difference in protein availability. I also feed a handful of hay in the morning, but they hay is less than 16%, so, if anything it would lower the overall protein availability. It could be that with certain breeds or under certain circumstances, the additional 2% would be a benefit, though I can see no evidence of that with my rabbits. I haven't actually done a "scientific study" with daily weigh-ins and a control group, but unless the extra cost of the higher percentage feed would have some obvious value, a few extra 10ths of an ounce at butchering weight is insignificant to me.
What I find to be of much greater importance is the quality of the pellet. We have cheaper pellet here than the ones I feed, (they also have a higher protein at 17%). In health, growth and palatability the pellets I feed are obviously better.