In the end, the timer is the only real thing giving these fights the challenge they have. Without it people could focus more attention on mechanics as they happen and even disengage from their rotation entirely to resolve mechanics if they wanted. As far as I can tell none of the bosses heal themselves and most of the mechanics are just making sure everyone in the party is placed correctly, so you aren't having to know what kind of abilities to use to counter enemy skills or effects nor do you need to worry about disabling or overcoming an enemies healing ability. It's literally knowing the dance and DPSing fast enough to beat the timer.