Not sure how you made those calculations, but spinifex is found in 20 out of the 50 states in the United States: AL, AR, AZ, CA, DC, FL, GA, KS, KY, LA, MD, MS, NC, NM, NV, OK, SC, TN, TX, and VA.
Outside the United States, spinifex is found throughout Mexico, Central America, and the West Indies; it is also found in the warm-temperate and subtropical parts of South America, the Philippines, and South Africa.
In addition, there are other species of plants in the desert and semi-desert areas that get caught in the undercarriage of vehicles. What we call "tumbleweed" is famous for getting caught under vehicles. In the Western United States, there are about ten different plant groupings that can form tumbleweeds. In early March, a couple of towns in Utah were inundated by massive amounts of tumbleweed:
View attachment 7850852
Video here is anyone is interested:
View: https://www.youtube.com/watch?v=DV6G8ipTSVA
I have no idea what percent of Grenadier owners will encounter spinifex or other dry grasses, but 0.5% seems like a low estimate, given the distribution of both spinifex (and other dry grasses), and the distribution of Grenadier owners.
But back to the Grenadier: the rear bash plate is a great idea, but its design might not be so great. In addition to potentially holding vegetation, it looks like - under certain conditions - it will also hold frozen snow. Perhaps the heat from the exhaust system will prevent this kind of build-up, but if not, it could be a nuisance and/or vector for rust. Be interesting to hear if folks in cold climates have had any issues with this.