I've only been main spec ret for progression for this tier (Holy before) and maybe I've just been spoiled by good healers but the only fight this tier I can think of that LH would be best would be Thok (using it on garrosh would be out of the question), but our healers were able to reach 30 stacks without much issue even on our first 25m thok kill so it never even crossed my mind. Maybe in 10m where healer options are much more limited I could see it coming into play more.
I mean as a mean's of comparison to use ele/enh shamans again, healing rain is a super strong, on-demand, low cooldown, doesn't require respeccing out of single target damage, heal from a hybrid dps. It just seems it would be much much more favorable to have shamans drop HR rather than having your ret's dropping LH.