The only time I really used LB3 is in progression content where we know we won't meet the dps check but we still want to go a little bit more to train some phase.
While we were learning P4 for A3S, I used it (or my co-healer) a lot if something went wrong.
The only time it was really useful was in A4N, when all dps died and when there was nothing else to do.
So yeah, I'm still pretty unconvinced by LB3 healer or tank, because they'll always be outclassed by dps LB3, if they had some bonus effect like "X% bonus damage or stat bonus during X time", they'll be useful.
I remember another mmo where the "ultimate skill" from the healer was a huge aoe on the ground that boosted your def+atk during 30 sec, and people freaking loved it.