The idea of rolling for defense has been discussed elsewhere. It's an intriguing idea IMO, but probably too complex for a tabletop RPG. For a simulation wargame, like Warhammer or something, having "saves" is okay, because one group rolls "hits" but there's nothing that the defender can DO to affect that aside from saves. So you roll hits, then you roll saves, then you remove dead models. It's pretty simple.
RPGs just work a lot differently, at least under the standard model. Armor decreases an opponents probability of scoring a hit, then you get randomized damage, applied against randomized hitpoints. Having another roll in there to negate a hit just adds an extra layer of complexity and probably prolongs combat a lot.
I think if you were going to do it, you'd need to totally re-write how armor works, and probably move it in a much more wargame-like direction. For example, you make an opposed defense dodge roll and add your Agility bonus (- any armor or weight penalties), then subtract that from the opponent's to-hit roll. If they succeed (either a static DC like 12 or maybe a moving DC for circumstances, fatigue, lighting, morale, whatever), roll damage dice, then you get some sort of armor save against the damage... maybe each piece of armor has a certain damage reduction die that you roll and subtract that from the damage done.
This would make it viable to have a lightly armored or even no-armored warrior, particularly if you had a high Agility, but still provide some strong incentives to using armor which exist in "reality."
But it would make combat resolution a whole lot more involved and, probably, longer. I don't know that is a good thing.
Hamun Ry (Wiz 4)
Str 10 Agi 15 (+1) Sta 11 Per 11 Int 17 (+2) Luc 10 (Unholy House). Align: C. AC: 14. HP: 13. Melee +1, Ranged +2. Crit: d8, I. Save: Ref +5, Fort +3, Will +4.
Spells: 1: Choking Cloud, Comprehend Languages, Detect Magic (odd crystal growths), Magic Missile (mirror images), Runic Alphabet (Mortal) (ravenously hungry), Ventriloquism (rain of frogs)
2: Detect Invisible, Levitate (extremely difficult, d14), Mirror Image (20% chance to raise/lower luck by 1d3 points).
Equip: Ring of the Sand Djinn: +2 AC/saves, Invisibility for 1min/spellburn point, or unmake for great, unknown effect. Padded Armor, Longsword, Longbow, quiver w/20 steel-tipped arrows, 10 silver-tipped arrows, backpack, spellbook, quill and ink, sturdy parchment (10 sheets), 5 days rations, high leather boots, belt w/ belt pouch, gray robe, dark gray hooded cloak. Also carries 3 small mechanical toys: wind-up mouse, wooden puzzle cube, small jewelry box that plays a little tune.