In my thinking, it's the middle that's most susceptible to damage, because you don't have the tires handling the weight in the middle. Front and back only ever handle half your truck, while the middle might have to carry the entire weight a moment or two... The most bent-up skids I've seen were thrashed in the middle...but my experience is limited compared to some here.
What you tink Taco?
So, from what I've seen, well, let's set the stage a bit.
Now a days, I view armor as not really for one obstacle or one trail, but as a overarching protection "plan" for the individual and their wheeling style.
So from what I've seen, most, not all, but most, kinda fall into two main categories, then there will be little subcategories off these, and cost is always the number one factor.
Both are people who want to adventure, take the family out, discover new things, not worry about damaging the underside of their truck and being stranded in the middle of nowhere with the only difference being:
Group 1: Hits the front plate, and decides to adjust, weather taking a better line, or bypassing, or something other than just pounding through it
Group 2: Looks at the obstacle, and decides they are going to get through it, doesn't matter how, drag their way over, winch if needed, scraping and banging.
I guess what I'm trying to say is take me, I generally take a bit more hit to the center of my truck than the front, but I fall in group 2, and while I would want stronger protection in the center, my style means that I probably will hit the front also, and probably not gently at that. Not to say I beat my truck, far from it, but I want to know what's beyond the obstacle in front of me, if my truck can't handle it, i'll make it stronger, and try again till I make it.
Now take my brother, he tests everything off his from plate, and he has a double cab short bed tacoma, WAY longer wheel base with so much more potential to slam the center of his truck more than any of us LX/LC guys ever will purely because of wheel base and break-over angle. But he falls in group 1, and regardless of his vehicle, his style, really only requires him to have the front be 3/16".
Funny thing is, we've always finished every trail together with no damage (only talking about my production vehicles)
I hope that I made some sense with that.