I'm just going to say this as a long term FPS player that did a lot of time in Quake and other twitch shooters... Removing half their life is better than none of their life. The ideal is obviously a headshot for a quick kill but in a team environment, you're rarely involved in duels. The best tactic, by far, is to provide supporting fire. Your Soldier 76 only has burst damage from his rockets. But a sudden drop of 150 health on his target can easily allow him to finish the kill. Obviously, snagging a one shot on a healer (especially Mercy) can be game-winning but at the same time, dropping all of a DVa's armor in a single shot just as she drops in makes it that much easier for your offense heroes to eliminate her as a threat. DVa is a particularly massive nuisance for most offense heroes because of that large armor pool of hers reducing damage by as much as 50% from some heroes. Comparatively, losing 5 damage as Hanzo or Widow means almost nothing.

Additionally, in a game like this where there are heroes that are incredibly effective at dealing with certain threats, sniper dueling is fucking stupid. I outright never bother dueling other snipers because that's a threat that can be far more easily squashed by a Winston or equivalent. Spend 30 seconds fighting the other sniper...or assist the rest of my team in securing 5 kills? I'm going to side towards the latter every time.