I'm sorry for getting pedant, but what do you mean by "need?" Need for gaining skill points? Or bonus? What you are saying makes perfect sense in the real world, but are you saying if I do Piggy In The Middle drill for one player (which obviously should be impossible,) he doesn't gain anything from it? Because I've done, and he does.
Or do you just mean you should make sure every drill you pick has some skill for every player that takes part in that session. So for example, if you train 2 ST's and 2 GK's at the same time, you shouldn't pick Slalom Dribble. That I agree with of course. But if I train 2 GK's I don't think I "need" any outfield players to gain Reflexes & Agility doing Shooting Technique. Granted, for GK's I haven't actually tried this. But other players appear to be happy training Set-Pieces and Pass-n-Shoot etc alone.