To summarize, IF is not useless. It's great, expecially for people that use medium and heavy armor.
Look at it like this:
An 21/15 build has 56 HP at level 30 with no IF.
With 7 IF, you got 70 HP. That's +25%.
In any competitive pvp game i've played, 25% extra hitpoints is huge. cRPG is no exception.
(God, i miss having IF... the curse of being cavalry
)