In D&D 3.x there appear to be only two types of armor <snip>
I think that depends upon the group you're playing with. Some people always min/max, some never do, and most of us are somewhere in the middle. To me, it sounds like your group(s) contain mostly min/maxers. Not that there's anything wrong with that...
Groups with players who emphasize roleplay a little more look a little different...
For example, our current 8th level group in RttToEE ranges from unarmored to Platemail...and very little of it is magic, BTW. Our main min/maxers are the Wizard (who's spellbook most min/maxers could probably recite in their sleep) and the Dwarven Fighter/Cleric. Only one other PC wears heavy armor, and one wears chainmail (mine).
In my last campaign, we had only one minmaxer (a Ftr/Wizard by the same player as the Wizard above- only multiclassed due to campaign pressures). Most of the other PCs wore Leather or Studded Leather, and the Paladin wore Masterwork Fullplate, but she never had it enchanted, despite many opportunities.
Personally, only one PC I've run to date in 3.x has had either armor you mention- and it was a mithril chain shirt found in the course of the adventure. I
have designed PCs that would use them, but haven't played them as yet.
While there are some good ideas on this thread, I think they are solutions in search of a problem. I mean, who cares if your PCs only use 2 different armor types? It just means you can equip your BBEG & his minions with spiffy armor that they'll probably ignore...