who says about lying?
when you train... yeah you train by position... but how many players are trainable in that position???
if you train only center... but you have (for example) 6 players playing 48, 50-55, or up to 65 minutes a week (depends on what is best)... they all will be trained... yet, the development would be the same (give or take... and of course, this is IMO) as training 3 positions with 2 players each position.
and of course... more development would be for the guy who played enough minutes, and him only (1 player playing 1 position) than training 1 position, yet having 2 players playing in that position. (2-3 players playing 1 position)
that's my point. and so, players matter.