While this works with ppl you KNOW it completely fails if you play random ppl.A friend of mine a while back started to get back into 40k and he had troops but no vehicles or anti-flyers so when I started playing him I resolved to take a footslogging army as I knew that he had no anti-flyers and would not be able to cope against a full mech list. Nothing in the rules made me do this, but I chose to do so. Another opponent of his took his usual list that included tanks and flyers. The mate in question didn't mind either way, but could have got quite upset that what I thought as being responable someone else didn't.
Now imagine you play a guy at - say - hobbycenter X. You provide a reasonably allcomer list no real hard elements in it, far away from optimised turnament lists. As it turns out "the guy" doesnt have a single heavy weapon in his list and cant do nothing against your only 2 tanks. Now he starts yelling at you how unfair and cheesy your army is.
He has no valid point. But in his mind, its still unfun. Thats where universal rules are needed.
You can bend the rules all day or ignore it completely when playing with friends. But not all peoply play only against friends.
In our gaming group we restricted allies to the original FOC (you still have to pick the "must have" units but you wont gain additional slots) and banned special characters.
But thats OUR gaming group. The entire rules section of this board would be basically useless because every answer would be "doesnt exist in our games" or "we houseruled it"