Players trained via friendly cost 0 green. You just play matches with 0% condition while training this way (if it impacts results just treat it as a throw away season). Green packs are used to power-train defensive players and others not getting MotM skill point. So economically speaking, training this way should help improve your defense over the long haul.

In this game, high powered offense alone will win you many trophies. You can hide a mediocre defense.