>YDについて
公式のガイドでは
What Does YD Do?
On a more specific level an improved youth development gives you the following improvement, increasing your chance of getting quality young talent significantly: The youth players spotted by your scouts (1) have a better chance of having better average hidden skills and skills when they arrive and (2) you get more players the higher your YD level is. The players have the same quality no matter how many you get. Here are the number of players received at each level:
とあり、
(1) have a better chance of having better average hidden skills and skills when they arrive
とあるので、一応、若干の能力底上げ効果はあるということだと。
が、平均が幾ら高くても「平均以上の選手」こそ欲しいわけで、結局最後は、運。その意味では「人数増」=確率増なので、重要ですね。