i dont know for sure either… but i have some great players with low stamina and i am training them on stamina…
depending on the tactic you play, a fast pace or a slow pace, they will be tired much faster or substituted faster.
also, i notice that players with low stamina, if they play large minutes, their overall score is significant lower than when they play only 25-30min…
so for me, it is worth to train them on stamina.
hope it helps.