So what’s the deal w stamina? Does it affect game play or not? I’ve read arguments on both sides.

It Affects it

My understanding is that they play less plays in the game with lower stamina. In retrospect, I think not paying enough attention to this in the past was the fatal flaw for some of my better teams.

I think it's like a multiplier against the ratings. For example, let's say you have an OL with 90 STR and 90 TCK, a player with lower ST will lose effectiveness faster. If he's in the game at 90%, it would be like his cores are 81-81 (90 * 90%). In that case, you would be better off having a player with 85-85 at 100% effectiveness.

That may not be right, but it's the way I've always thought about it.