No it is not. It helps all.
Lower league will finish with D-league training sooner, as they need to bring them to lower level, but all could use and improve using the D-league.
Of course that (as I've already wrote) using the D-league costs money.
Of course that (as I've already wrote) the player salary should be paid.
Of course that (as I've already wrote) it should be added to the feature that will not allow users to sell (or fire) their first draft picks at their first few seasons.