Agree 100%. This is the most asinine rule of all time. So the theory is that this makes the players get acclimated to the national team rules or FIFA rules or whatever. WRONG. A player is either ready to play a full 90 or not. Maybe they're only physically fit to play 25, 45, or 60. There is no strategy involved in this.
The strategy or acclimation is for the COACH to get used to figuring out his or her lineup according to the limitation of his or her roster. If this is truly for development, it's for the development of their soccer skills! At this point of their development, who gives a rats ass about how many times you go in and out of the lineup. Again, any strategy with limited substitution rules are for the coaches not the players.
And to MarkM's point, he's right, this wouldn't be an issue if rosters were kept to 15-16 players. With limited substitution, and rosters of 20+, it may be driving players out of DA to get more minutes elsewhere. But isn't TRAINING the most important point of DA? So if training is the most important aspect, have more flexible substitution rules so more players get more play time and stick with DA. Then you have the type of roster to get the most out of your training, which is most important for development at this point in their journeys.