Support is so important in this game, and what ultimately pushed me from S to S+, was a change of weapon, to the Zink Mini Splatling, and playing support. I have won so many Splat Zones matches by just keeping the enemy back with disruptors and covering the zone, and area around it, so the enemy team can't push the zone. In Rainmaker I will focus on the rest of the map unless my team needs me, then I flank the enemies, we take the Rainmaker, and from this map control we can win. Tower control I play similarly. This doesn't have to come at the cost of K/D, but sometimes keeping enemies from objectives is enough, they don't have to be splatted always, just distracted long enough to get a good lead, but obviously stopping the Rainmaker and tower are required too. It is a balance of supporting your team, and splatting the other, but to claim support is not a key role is just ridiculous, support, in the form of map control, increases your team's offense, and reduces the other team's offense, making everything easier.
Edit: To clarify this doesn't mean I always have a bad K/D, last night I had games of 10/4 or 9/3, but I've also had games of 3/9, and felt I contributed the same amount to the team. K/D is too simple a metric to determine your value to the team. Bad snipers can have a good K/D but lose consistently because they don't contribute towards the objective.
Ultimately, random splats are worth little, the enemy team will just wait a couple seconds. Stopping a push towards the zone, or stopping the rainmaker and at least one other person, are worth much more, and support is what makes these kinds of plays possible.