My guild used a DKP (dragon kill point) system for loot distribution, which was fairly common. Basically you earnt points for showing up to raids. When items dropped whoever had the most points and wanted it got it, spending some number of points.
At one point an item was left to rot as no one wanted it.i came to the raid late and passed the corpse. I looked on it and saw the item. Now it was nothing I'd spend points on but I did want to experiment with it so I took it. Someone saw me and an officer who I didn't get along with kicked me from the guild. The guild leader was basically inscribe by that point so wouldn't overrule the officer and that was that.
So I was technically in the wrong but, like many things like this (in game and out), it wouldn't have mattered if I was in the in crowd but I wasn't so it was a valid excuse to get rid of me.
As for playing 16 hours, in the fueled I was in that was normal.
As for being in demand, it's a bit more complicated. Some classes are more desirable than others. I was primarily a rogue, which at this point in the game was largely useless in raids except for scouting and CR. The class balance shifted from expansion to expansion. The next expansion made rogues desirable again.
Plus I HATED the current expansion and had had my fill of raiding pretty much so I didn't pursue other guilds but it would've been toughg to find one with room for a rogue.
Bear in mind that raid sizes were limited. At that time it was 72 people for one expansion and 54 for the next so there was pressure to reduce guild size anyway.
Most guilds operated as close to 100% capacity without going (much) over as possible. To go lower was to be under strength. To go over was to force people to sit on the sidelines, which is no fun.