Stamina is absolutely vital for any players who use playmaking. For inner midfielders it's not a secondary skill, but a primary one together with playmaking. One of the most common mistakes by playmaking trainers is to slowly increase stamina for their players. Playmakers with anything below solid stamina are a waste of resources in a league or cup game.
If a player does not have enough stamina, a serious drop in their playmaking contribution will be noticed, particularly during the second half of matches.
Training stamina is not very popular, as it may drain the form of players in the squad.