Early in the game like where you are Volcanic if you looked at the youth players value compared to how much cash you could get for the same amount of tokens you would see that purely from a financial stand point its worth it.

Later in the game when cash isn't that important there are more active players so bidding wars are fierce for high value 18yr old players and any position with a special ability usually goes beyond round 20 anyway and your not guaranteed that they'll train fast anyway. They may be a dud from a much higher lever players youth academy. Whereas if they're home grown you know that they will train fast so you know that they'll be in the team for a long time.