I think every team need to PT 5-6 18yo for the future bcz simply you can't buy whole squad in a season .. but more than that is waste of green and red packs ..
ex: i bought an 18 yo 5* for 4m (in level 3) .. PT to reach scout , he will be 5* again next season in level 4 but you wasted 50 or more green packs (plus GOD knows how many reds ) .. while you can buy new 5* 18yo next season with better development rate for same money .. do you see may point.. my new 18yo will improve faster than my Power trained from previous season