Yeah, but look at it in a different way. It trains the whole team quicker, true. But even if you want to have your team composed of players you trained yourself, you don't train them all at the same time... Specially because they're not 18 or 19 forever, so you need to use the time when they're most eager to improve to give them most of their training. With team training you'll spread pops to the whole team but I don't think you'll make a big player in the midst of it. Not to mention that different positions require different skills...
Maybe useful now and then, but not a reasonable long term training plan, at least from where I stand