If its an alt run to gear ppls alt and some1 comes on main, then its fair game.
when we did alt runs, ppl were free to come on mains, but mains wouldt get prio unless it was WF/socket weapon or trinkets that was an upgrade for said main.
Also, i REEEEEALLY hope they will make it so we can see if an tier will give WF or socket, will make life as raidleader/Masterloot so much easier