I think rolling makes knockdown mitigation moot. I know I personally still wouldn't care to put points into it, since it's not that difficult to get a very high rolling % nowadays. Weaker knockdown weapons already have a meager 7.5% chance to knockdown on the head and legs, while cav bumps are going to knockdown each time. That leaves a small subsection of weapons this would be useful against.
Because IF is universally beneficial, I don't think a 1:1 ratio in increased stats compared to the other skill points is a wise idea, but I would like something better than 1:4 or 1:5. That's why I believe that IF aiding in the dropoff in damage of held strikes would be helpful. In the past, held strikes were changed from a constant multiplier of damage no matter how long you held, to a low point of 50% (iirc) after a few seconds. That's why I believe IF helping in this area won't cause an imbalance. 0-3 IF builds may need a small penalty for how long they're allowed to hold for great damage. This would make it so 0IF / agi builds need to time attacks even better with the limited holding ability. Because they release attacks faster anyways, I think that's fine.
It's not a dump skill. STR and IF combined can more than double your HP pool. Suppose I have the choice between 24 STR 7 IF 8 PS and 27 STR 0 IF 9PS. Unless I plan on gaining further levels, the 24 STR build is massively better.
I still find it silly how one would think IF isn't a dump skill then proceed to compare 7 IF points to 1 extra PS. There are situations where you have an extra 3-5 skill points on top of the IF. For instance, let's say you were level 32, 21-18, 7IF with 3 extra skill points. You could stick with the 21-18 build or you could be 24-18 with 3IF, leaving a difference of 5 measly hp. Losing 4 IF instead of 7, all of a sudden that 21-18 build looks pretty crappy in comparison. That's because it takes too many IF points (dump skill) to become equivalent to other skills.