What I mean by 'need' is that the player has to be there. Else, another selected player is likely to be involved in the secondary role, and will likely make gains in attributes that are not wanted. But, it seems that some player still make some 'unwanted' gains, probably due to the system not allocating the roles properly. Take into consideration that I have not thoroughly tested this. It might well be that the selection of a player for gains in particular attributes related to the training drills selected is just random.