Indeed I would gladly give up any Penta for a W any day.
If a kill is 100% free, yes let the adc take the kill but not if you don't have vision or it's "risky".
Supports getting early kills in lane early in game just means more
and more items to protect the adc so later on the adc has vision to get the kills without the help of the other lanes.
Top players
down to bot and pick up 1-2 kills early or whenever just helps them scale faster and getting more items to peel for the adc for days which in the long run will let the adc have the chance for the triple kills- Penta kills opportunities.
Like you said Penta Kills are just for show and once you get 1-2 before you don't care as much but the W/L stays in your mind forever. I remember the games I lost and won more so than the Penta kills.