There are several reasons there are still melees in raids, a lot of mechanics require melees or are just more convenient for melees (kick duty, soaking stuff, sharing damage in melee range)
You also have to consider loot distribution, you'll probbaly have 1 plate tank, 1 hpal, you want some more plates to get balanced item distribution
Wow is the kind of game when you'll take a good player over a class (most of the times in non extreme situations), very few guilds have the most optimised raid comp because they'd rather have players they consider good
Also it happens melees do way more damage than most ranged, which was the case this tier, Arms warriors destroyed everything on cleave
Despite all of that i don't think there is any advantage playing a melee on any fight, you're penalised by every movement, more exposed to aoes, the only benefit is you most of the time have less mechanics to handle than ranged