If you think IF isn't useful you have no idea what you are talking about. Which is understandable. I can gather you play the long axe often? No need for a build even remotely competitive when using that, you'll dominate anyway. Fact is with a good armor choice, investing in more IF instead of another agi tier is going to make you survive usually between 2 and 6 more hits. Agi is never going to make that much of a difference. The combination of high armor reduce and IF investment easily transforms someone into a damage sponge with only marginally lower acceleration, speed and wpf than a build that converted those IF skills into agi. Overall, IF is a much better skill than shield skill or riding as it remains fully effective under all circumstances and doesn't stop being effective above a certain item difficulty threshold.
I stopped reading your BS at the bolded text. I mean, are we talking full +3 milanese tincan vs level 1 peasant? If so, sure... having the whopping 12 or so more hit points might allow you to survive that many more hits, assuming the poor peasant can even manage a solid non-glancing blow. But in practice? No... having a few more points in IF will never allow you to take that many more significant hits. Heck, almost no one even survives 6 solid hits nowadays, not even tincans... some IF is never going to change that.
And for the long axe dig, I have been playing my alt an awful lot recently in order to grind out some loompoints, and I have been placing just as highly, if not higher on the scoreboard when going 1h and 2h compared to my polearm main, which uses far more than just the long axe. Using my lower level alt is what influenced me to make this post, since having 3 less strength and 4 less IF made almost no impact in how many hits I could take. With that large a discrepancy in my IF stat, I could take maybe, MAYBE 1 more solid hit on my main. If we are counting glances as hits, well then I suppose I could take upwards of 10 more hits! But that would be retarded to... oh... wait...
The big problem that I find with agi based builds is that they break animations for me at 150-170+ wpf. Perhaps lowering the speed of everything(arrow speed, movement speed, wpf, weapon speed) at a particular ratio and then increasing the server speed? Other than that all you can do is buff IF ALOT so people actually use it(I haven't had it for 2 years) or buffing PS so higher str builds hit harder then the agi builds with their huge wpf damage enhancement.
I don't find that broken animations are limited to agi builds for me. Even with your 3WM build your 2h spam still insta-swings half the time.
I am fairly certain that it is just server lag or broken game mechanics that have been around forever now.
Also, I believe that the main issue with agi builds hitting almost as hard as str builds is the speed bonus they get. Looking at damage calculators, wpf barely increases your damage output (compared to PS, anyways). However, something that is a lot harder to calculate is how much damage you get per athletics speed increase (assuming of course you know how to use that speed bonus). I am fairly certain that with your build you are not only fast, but have one of the highest damage outputs available, save for maybe a full PS build.