Healers that do this are nothing but plain annoying (and bad). There is quite literally no reason to do this in normal dungeons or 24-man content. I can maybe see what Sebazy was saying about doing it to optimize DPS in your own savage raid group, but then you would have discussed that with your tanks first vs. doing it to random DF tanks.
I know I'll never stop people from doing this, but every time I see a healer doing it in normal Omega or Rabanastre, I roll my eyes and immediately think you're a bad healer (and about 90% of the time, I'm right and they are bad).