Honestly in SL, it really is always the best choice. You don’t have to have it run the full duration to get value, and it is outstanding for punishing positioning mistakes. Although you could simply Cocoon the Ming or Tass, I will more often use it as a counter, rather than as an engage.
If their Muradin or Mei dives wildly into my back line, they get a 7 second time out to ponder their life choices. If their Tass pops Archon, he will lose at least a little of its duration while his team mates break him out, or I will put enough pressure on them without their heavy damage dealer. A Dragonblade Genji is a non-issue. Against Ana or Morales, save it for their Nano/Stim target. If the enemy tank is an ETC, good luck with getting a Mosh Pit off.
While traditional targets are the healer, main damage dealer, or sometimes the tank, Cocoon doesn’t have to be used that way to be effective, and I have yet to see a comp where I regret picking it. As with all things, timing and target selection is key, but Cocoon isn’t simply good for only turning a fight into a 4v5.