I think the thought was to make stamina important for the 2nd half of games, but however it's worked into the formula, it gets lost or overrided a bit by other factors. I do think there is a small boost to your players via the "end game rating", but with all the randomness in the game, the outcome of a high stamina is really not significant. I will say that my team performs better in the 2nd half of games since all my starters were over 10 stamina, then they did a season or two ago, so there is a slight benefit in my opinion. But that could also be that my players skills have gone up too. Personally, I've always liked the randomness, otherwise, we couldn't ever simulate the Rams beating down the Saints last season