It also reduces analysis paralysis for the DM (well, I have a bonus, I guess I should use it), gives the bonus to the tactically worst target (the target who has the most time to recover before their next turn) which should increase perceived threat more than actual threat, and encourages spreading out threat (unless things are really important).
We don't force responding to a PC, we encourage it.
As the N in Elite(N) only meta-game wise (The advice is that against a group of 5 PCs, use an Elite(5) to make it a challenge; it isn't Elite(*) where * is the number of foes. The difference is subtle) scales with the number of PCs, bringing in chickens or mooks to the fight doesn't make the monster more effective. The monster if they act on a chicken or mook turn only gets the advantage against the chicken or mook; they probably should save it for PC turns (unless they want to smoosh the chicken or mook!)
It also encourages great drama. You hit the monster, and the monster smacks you cross the room (or tries and fails). Which makes each player's turn a somewhat personal conversation - PC does something, monster responds, next PC.
Also, as we (effectively) factor it into the Elite(N) power budget, it makes the "focus fire on one PC" option ... less optimal! Weaker even. And focus fire being optimal leads to, well, boring combats.
This also makes abilities like Sentinal and position based OAs more fun. Because the DM is encouraged to not focus fire, abilities that push the other way get triggered more often.
This is all very good stuff. Nice work.