To put it simply, first is that a lot of people don't really care that much, and second, the high-DIN bindings tend to be built sturdier, higher quality, and last longer, and more people than you'd think run 18 DIN bindings as low as 8.
*Paragraph ranting about ski bindings and how much they suck at protecting from ACL tears*
To be fair, ski bindings are systemically flawed when it comes to ACL ruptures, so people who crank DIN's *almost* kinda have a point. You can blow a knee at virtually any DIN setting, its just more likely at the higher ones. DIN/ISO standards regulate release values to protect against tib/fib fractures; when these standards where written, they were the most common ski injury, with ACL ruptures being a somewhat rare event. As better equipment helped us ski faster and bigger, and with other injuries not as common anymore, ACL tears became more prevalent. To protect against ACL ruptures, you need a lateral heel release. As it is a difficult problem to solve technically, and there is no requirement to do so, bindings do nothing about this (with the exception of knee bindings, which aren't DIN/ISO certified and pre-release). If you look at your top of the line Marker Jester Pro's or Tyrolia's, the heel *might* come out laterally if you get lucky and the plastic heel pushes out of the heel cup by pushing the binding heel piece backwards against the forward pressure, but isn't really the way the bindings are designed. On a Pivot, you have even less luck, with the side-lugs that hold the binding heel piece physically blocking the boot heel from coming out laterally. This isn't a knock on Pivots, they're solid bindings, and have advantages over Markers for instance (toe pieces last longer), I'm just saying the rumor you hear sometimes about Pivots being better for your ACL's is total BS.
As far as the argument to "run a binding in the middle of its DIN range", I have not heard a single argument for why, other than "Its designed for it". My guess is that you want your retention to come more from spring stiffness and elastic travel than spring preload, (you cant change spring stiffness, but you can add preload to increase DIN setting), but that is a guess, and only explains why you don't want to max the DINS, not why you shouldn't run them super low in the range. If anyone has a legitimate technical explanation, I'd love to hear it.
In summary I don't think you should crank your DINs too high, but I do think there are legitimate reasons to get higher DIN bindings that are built better (within reason). If you run an 11, I'm not aware of any downsides of getting a 6-16 or even an 8-18 binding.
