I don't think I overstated my case at all to be honest., if such a change was made I predict a dramatic drop off of anyone taking the feat with your noted exceptions of natives such as Gundarakite Rebels or Fighters who eventually run out of things to use the feats for.. I am totally for the non-stackable change in standard rulesets but in a cast of hundreds I prefer there be more level of standard deviation and permutations that require fore-thought to achieve. Our main disagreement as I note, is that you believe currently it is a NO BRAINER that characters take the feat here. Whereas I see it as somewhat of a choice now at least if there are other feats you would like to have (I don't disagree its one of the few feats that benefits any character class or build, thats true), but if they did not stack would be a NO-BRAINER NOT TO waste a feat. I find it quite humorous the different insight, neither of which I believe is right or wrong. I simply can not see your view because its not how I play. Of the 10 characters I have played here that reached a level high enough to select the feat Improved Critical, I have two that did take the feat. I WANTED the feat on every single one of them, simply most did not have enough to have it AND the other things I wanted them to have and I had to make a choice. I would greatly welcome that being one less feat I would like to have because it solves a bit of the dilemma of choice, but I do wonder if that is a good thing.
For example my Drow Cleric hits like a truck with Divine Might, varnish, Divine Favor, base damage, strength damage all but varnish multiplying and she only has Keen Edge spell items or a Mage's help. She does not have the Improved Critical feat and I don't miss it other than the nagging feeling she could actually KILL THINGS FASTER if I did take the feat.
She is still on par with my equivalent level non-casters damage wise even with the cleric's insane advantage over larger sample sizes, simply because they attack more often and hit on criticals more often which closes the gap a bit on the 5-9 extra damage on every single hit, because they do have the feat which makes them critical hit as is possible in the rule set and shes does not.
In summary, removing the stacking nature would be tantamount to removing that extra obtainable level which is not a terrible idea per se, but simply has the non-disclosed and untidy side effect of making Paladins/Clerics hit the hardest in any sample size which is already likely true but at least only for the ones who have built like a fighter and taken improved critical. At least in that scenario they outpace a fighter but they had to give up a feat (which as I understand it you believe is not a sacrifice so fair play to you). I just see the current status quo as having the tidiest and fairest permutations and I prefer it over seeing anyone with whetstones reaching the highest range. I repeat I would be in favor of a weaker version of either/or, but I just think its best they DO stack so as to reward the feat by granting a unique extra obtainable level of sharpness over anyone without the feat, even if the advantage is as small as it gets (5%)
So my "case" is summarized thusly : "Yeah, thats a neat idea, but in the interest of balance here is some things that spring to mind since this is a thread in the Gameplay Balance section and I had some things occur to me regarding such a possible change" Its not an endorsement or denouncement, there is very little real impact on gameplay outside of competitive PVP of which I do love but rarely participate in so I don't have a dog in the fight.