One of the strengths of d&d combat is that there are so many facets of it that model cinematic aspects while the holistic system does not bog down in detail.
The critical hit serves a function that draws the players attention to a significant event in combat that can alter tactics. E.g. the BBEG crits with a *3 weapon on the fighter drawing gasps & so the cleric & fighter then plan their actions to rendezvous to redress the situation, etc... Of course without augmentations, this is pretty rare occurrance & with augmentations it still is a minority except for the 15+ scimitar/rapier/falchion. At 15+ the falchion fighter probably threatens on a hit about 40% of the time because at around the mid-levels onward the primary fighter's BAB start to out-strip the average AC as the secondary attack grows in importance, i.e. the primary BAB fighting type starts to hit on numbers lower then 11. Regardless, the 15+ critical has become fairly common, common enough I think to represent the crit skilled archetype. With 2 attacks there is already a better than even odds that they are at least going to threaten a crit, so already they player's character is drawing the crowds attention.
Admittedly this is all academic because I never experienced it in my games, but the 12+ crit has some fairly obvious effects. The majority of successful hits occur in the upper range of the d20, if we bring the 3/4 BAB classes into the equation. We've got such characters pretty consistantly generating critical threats & hits so much so that the words 'critical hit' is becoming common. But we've still got the *3 (the 19+/*2 weapons are not so important) generating criticals which are of greater significance to the tactical situation as they suddenly dump a whack load of damage.
Without going further in this examination I see 2 problems here. The 12+ crit is occurring with such regularity that there is a case building which is strong enough in my opinion, for it to be replaced with a flat bonus. The criticals are beginning to occur more often than the standard hit, which perversely is then beginning to be the focus that influences combat: i.e. "drat guys, I only did a standard hit on this enemy..." The base description of our crit expert has become one of constant sharp attacks & where the successful standard attack is against the norm. It is a classic case of untrammelled satiation where the lack of limitation distorts perspective, 'to hit' is supposed to be 'to succeed'. The second problem is merely a hinderence to communication, that of the significant *3 event being lost in the chorus of critical hits.
****
As an afterthought Argo, I pose you with this question. Would you object to a 3rd or 4th critical augmentation that could potentially improve the threat range to 9+ or 6+ respectively? If you would object, on what grounds would you justify it?