I agree with both of your points. This idea would really be just to add a little additional gradation in armor classes - who knows, something between leather and chain might be really nice!
Re: Capristo's point in particular: One way to think about (resolve? change? idk) the "changing armors" situation could be through a combination of different dungeon environments, and different player stats. Current PCs have a specialty (i.e. some PCs are born needing less XP to level up arm/leg strength than agaility/dexterity, and vice versa). Someone with much higher mobility than strength would, naturally, want to favor a lighter type of armor that doesn't impede them as much. This could further be tweaked by having certain dungeon environments favor lighter (slow, heavy hitters) or heavier (lots of fast, weak attacks) armors. Thus if an agility-centered character wanted to do a dungeon full of bats, he might do well to wear ring (or chain) mail instead of simple leathers.
Just a thought/option.
Thinking about it, though, *if* this sort of addition is ever to be implemented, it might be best to do it *concurrently* with armor balancing. Additional armors will require additional balancing - both for the new armors, to make sure they fit the curve, and for the old armors, to make sure they don't become obsolete.
Armor balance is such a big issue (which probably ought to encompass shields as well!) that I doubt anything definitive could be done for the next release.
"Put more stuff in the... thing where... more stuff goes in."