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 )
you can actually have if with that build if you plan your build for the level 31(obviously you don't have to be a shielder hybrid)
21 str
15 agi
7 if
7 ps
3 shield
5 ath
5 riding
5 wm
cav,70 hp, good athletics good damage and good wpf while dismounted(it's the build that I'm using atm)
however turning back to topic, it depends on your style of play
let us take as an example the build I made above, you can basically do 2 thing
-if you get if you will get 14more hp, that is 25%more of the base hp and will be 20% of the resulting health bar(basically if you divide your health bar in 5 pieces the last one is the if bonus, and it is good also for showing when you would have died without it)
-you can convert the 7 if to have 3 more str and 1 more ps(0.7x3=2.1% +8%=10.1% base damage bonus)+3(5.3%)hp
you obviously can do similar comparison with agi and bonus speed by wpf, however 10-15 more wpf on melee means really nothing trust me
so it all comes down to you, converting means +10%damage and +5%hp, not converting means +25%hp for this build, but similar calculations can be done with any build with if, I'd say if has always been very useful and with the new soak values it's really worth it for any melee oriented player