I think its a valid training strategy, especially if you're looking to manage minutes well and keep your players in good gameshape
Trying to train 3 players in single position training means 48 minute game, plus starting them another game. In a difficult league, this will usually mean your flogging your player for 80mins + a week.
We can't all train gameshape :)
I do train 3 players in single pos though.. but their gameshape suffers as a result