I rarely use it, the only time I use IW is on running back from a wipe when I have Inner Sanctum glyphed. (sidenote glyphing IS and Fade for Heroic Garalon is kinda fun)
For disc:
Really shitty gear levels/running at ta'yak is all I can think of in pve.
In pvp I'd argue that it's better than inner fire in most situations, to kite melee (just make sure you get IF up before they get to you), vs casters (when line of sight'ing/mana is relevant) & basically any time you or your partner isn't in immediate danger to conserve mana (almost all of your pvp spells are instant so IW is huge).
Regarding disc/holy both are dreadful in pvp atm but disc is slightly superior.