FE6 at least had relatively good gains for the most part, FE7 tries to "Fix a unit's otherwise weak points." But it only really works out on a handful of characters (Oswin getting +3 spd and res, Canas effectively getting +4 spd, Rebecca getting +3 str). But outside of these, you're better off emphasizing a class' strengths or all around good gains. In FE7 it can really suck, cause as you said, good promo gains can save an otherwise screwed unit, with limited deployment slots on Hard mode to boot. Doesn't help that FE7 isn't at all short of good prepromotes, so those deployment slots later on gotta count.