Some comps literally only train healers, unless there is a mage on the other team, then they sit on it the entire time. Ret/dk/healer is probably the worst, from the dumpster leagues to 2700 they all play 100% the same way. Watching a rank one streamer lose to ret/frostdk/priest because the ret/dk can just sit on a target 100% of the time with no worries about cc, not even pop cds at the same time, and still force block in the first 20 seconds of a game, then a second a minute later, then kill right after that is pathetic, and shows the state of the game.
As a Subtlety Rogue I find myself constantly swapping targets and off CC'ing. Primary kill target in a lot of comps tends to be the healer (Ret Paladins are another favorite), although even then it can often be somebody else depending on how defensives are used and positioning/opportunity.
Nothing makes me happier than getting a 6 sec Kidney on an off target that's badly positioned, then following it up with a Sap and possibly a Blind without giving up much of any uptime on my target, and then Shadowstep/Kicking them when they finally get control of their character again and try to get a cast off. I can almost see them punching their screen as their partner dies.
Most people can survive a round of CC with their defensives and it's easier to just zerg a healer and lose if it fails than it is to cycle two rounds of CC or pressure to pop defensives before CCing. Especially with a random to cap with.
When there's more CC coming from the dps, its very difficult to sit on healers. With the nerf to CC and the addition of dampening, training a healer is often times the best strat.