Mass Summon was dumb, I was usually the guy casting it, then 80% of the raid took that. Then we'd get through trash and the other 20% would bitch about not getting a summon.
Now it's usually like half the raid flies to summon, the other half whacks it and waits. We've had people ask for summons into Siege on Immerseus before we start for the week. At which point, as a raid leader, I tell them "Fuck you, you're sitting this boss."
I don't expect that to change much.