Oh, I'm not saying that it isn't useful. It most certainly is. It just isn't "on demand" like, say, PI. BT is a bonus for casting PW:S, and as such really cannot be calculated into the speed of casting spell1, when you need spell1 to be hasted "now."
Where it shines is in burst, especially on a single target, for combos like PW:S->Penance->GH/FH. To throw out a PW:S for the primary purpose of gaining BT for your next cast time spell, however, I think is misinterpreting it's function.