Why is this a thing? Is it really EVER that serious? I have to assume this is only speaking of DF since anything midcore up doesn't even have these issues of "You must deeps" since it's a requirement to even stay on a team.

If it's a daily or random dungeon, seriously, who cares if a healer just stands there of the tank doesn't stance dance/optimize deeps? Is it suboptimal? Yeah, no doubt, but are they fulfilling their function? If so, you'll beat the dungeon, and any kick is gonna waste more time than if you just shrugged it off and kept playing like it isn't that serious (Which it isn't).