Questions about legacy league

Hi, just two questions:

1. I started playing a few days before Warbands/Tempest...and after doing some research, I've realized that a lot of the stuff in the core game were once league mechanics (chests, shrines, bloodlines, torments, etc). We already have this stuff in the core game. Whats the point of selecting only three at a time, that are consumable at that... Why whittle down to three, when we already have all of them in the game?

2. If you are playing with friends, how does this work ? If you have different leaguestones selected, how does the game determine which ones to use? Does it go off of the host of the party, or use all of them?

There is a pretty massive difference between randomly occasionally running into an exile, and being able to guarantee that the next zone will have 3 exiles, 3 breaches, and 3 shrines...

yeah my understanding is using these league stones creates a sort of supercharged version of the league for the zone.

for stone usage I think it is determined by who makes the given instance