I've noticed the same thing, where armour feels like it's improving from each stage so much, that you feel no desire to use the lower levels once you have the higher levels. Even the auto-upgrade-equipment feature just treats light armour as a stepping stone to 'the real stuff'.
I've got a half-typed up post saved where I'm trying to decide how best to present a variety of issues, but here is what I worte about armour:
Change the armour research system so that the three levels of armour research (leatherworking, armour, heavy armour) instead provide three different levels of improvement of three armour types, light, medium and heavy. These three armour weights function similarly to the three current armour advancement levels, except they are all available from the beginning. The three levels of research improve all three armour types along a three tier system (blimey this involves the word 'three' a lot).
So for example, 'Leatherworking' as a research skill is replaced with 'Basic Armour crafting' and provides three types of armour, a light (Padded), a medium (basic chainmail) and a heavy (coat of plates). As you continue to research, you unlock the next tier of armour research called 'Advanced Armour crafting' which unlocks three more armour types, the light (Leather), the medium (splintmail) and the heavy (Half plate). A further level of research provides another level of advancement.
Each armour type has its own genuine advantage and is all a genuine consideration, depending on the role of the troop type. Light armour weighs almost nothing and so works well with archers (to avoid a worse initiative penalty than the bow already provides). Medium armour weighs a bit so can end up with an initiative penalty, but is overall acceptable, making it fine for quicker skirmishers or basic soldiers. Each piece of Heavy armour provides an individual initiative penalty in addition to the weight concern (an important point, since this would provide a serious disincentive to use this frivilously) but strong defensive abilities. This level of differentiation would mean that armour wasn't just a stepping stone towards something better, but each different type had a genuine reason to exist, even in late-game.
This could apply for shields/offhand as well. The three different armour weights could each provide a shield with different benefits. Small shields could give a larger bonus to dodge but a smaller bonus to defense, kite shields could give a moderate benefit to both, while tower shields could give a small penalty to dodge but a strong bonus to defense. (you might notice I included the dreaded phrase 'offhand' above, for reasons that will be clear later.
Note, I'm not in favour of the armour actually providing stat bonus' unless it is magical. It makes no sense for a character not in armour to be slower than a character in light armour, because the light armour provides a dexterity bonus.