One of my friends suggested this at one point, when I announced my Cleric had an initiative score of "22" in combat once. I was in full plate, max dex bonus of +1, but I have a +2 dex bonus. He said I shouldn't have a 22, and that max dex bonus affects EVERYTHING.
Well, as soon as I explained to him "No, it ONLY affects AC", he said "Well, it SHOULD affect everything".
As soon as I told him how that would completely break high level rogues and monks, as well as EVERYONE'S Rfx save, he shut up about it.
Heck, if this game was all up about realism, you shouldn't be able to move much more than 5' per round in heavy armor anyways.
The max dex bonus is more of a game mechanic than it has anything to do with realism... it probably SHOULD affect RFL saves and initiative, as well as multiple ranged attacks per round (how can a fighter with full plate nock and release 3 arrows in 6 seconds??)
But this is D&D... the rules aren't always based in reality. Max Dex bonus is a check to prevent stealthy high-dex characters from getting a tremendous AC advantage. I could play a ranged attack rogue, who doesn't care at all about moving silently or hiding, jack his dex up, wear full plate, and then I'd have an AC of like 25 or so at level 1.