Yes, ideally you're supposed to have your DIN in the middle of the range. Of course, that's not always possible. For example, you'll have kiddy skiers that have a DIN of 0.75-1.00. Pretty much all junior bindings run from 0.75-4.5, so ideally, that's not the best binding for that skiier.
But again, bindings need to operate consistantly though the entire DIN range of the binding, otherwise binding manufacturers could get into a bit of trouble. Technically, you can crank up the DIN to the max if you need to, but ideally, it should be in the middle for the exact reasons that you mentioned.
As for the pivot helping prevent ACL injuries... not sure, but maybe a Rossi or Look rep would know.