IF as a whole isn't that useful unless you're planning on going a very tanky build, because of the obvious benefits you gain from IF and heavy armour, and currently as a whole STR is a lot weaker than AGI, it's easier to spam people down with a mass flurry of attacks than it is to whittle them down with large and heavy hits.
Obviously it's dependent on how well you can block, if you're an inbreachable fortress when you block, then an agi build is suitable as you can make use of the low health and instead trade it into damage AND speed.
Whereas if you're worse at blocking and better at attacking perhaps a primarily tanky build is better for you, as then you can sustain massive hits whilst doling out tons of damage.
Or, in the best of both worlds, you're an unbreachable fortress with tons of soldiers manning the walls, (great at both) which is a lot of people play balanced builds, or things close to it (21/21) (24/21) (21/24)
But most builds don't contain IF as it doesn't really give you much benefit other than a small amount of extra health, whilst great for people with not so amazing blocking ability, and a decent attack, it's worse for a lot of people who could make use of the extra PD, PS, WPF, ATH. Almost anything really.
Most people in C-RPG have a decent level of blocking regardless, so for the most part there's no need to play a tanky role other than to help your team, and given that most people prefer to kill shit instead of tanking hits, they choose the much stronger route of AGI. Personally I think STR needs a buff to make it a lot more relevant, but that's just me, I don't know about anybody else.
