It's not legitimate if the 12th man doesn't play. I think it applies to active contributors.
So, yes I agree you need to be careful in taking it too literally.
I still don't think it holds up. If we're talking about "active contributors", then we would probably say that Luke Walton is the Lakers weakest "active" link. IMO, he's not very good at all. He plays with a certain level of intelligence and knowledge of the game, but he constantly gets the ball stolen from him, his shots blocked. To put it nicely, every time he has the ball in his hands, I get scared for the Lakers.
I see the Lakers play against other teams whose bench players (their weakest links), literally make Luke Walton look like he should still be playing college ball. I think Tony Allen of the Celtics blocked a couple of his Luke's shots, as an example. Luke never got revenge. He got outplayed by a large margin. Yet, Luke's team, still won the series.
Do you contend that since the Lakers won the championship, that Luke Walton is the best of all the "weakest links" from all 30 teams?
If Phil Jackson played ME for 10 minutes each game, I would EASILY be the weakest link. But, the Lakers would still be able to win some games. They would win less games with me playing 10 minutes, but they'd still win sometimes. I just don't see how it holds up in reality. A team is the sum of all of its parts. If your parts add up to more than my parts, then you win - even if your worst part is REALLY BAD.