Thing is, the non-plate specs give you something other than a bonus.
Hide and Chain spec remove your armor check penalties as well as increases your AC by 1.
Scale removes your speed penalty as well as increases your AC by 1.
Shield spec increases your Reflexes as well as increases your AC by 1.
These three feats are already balanced towards each other. AC +1, and an additional bonus.
Plate only increases your AC by 1, but if you're a two-handed paladin build, this is your only option to increase your AC. But then, you're -not- a high-defense build so you're not too torn about that anyways.
You shouldn't be getting huge AC by feats, beyond upgrading your proficiencies.
Besides, the idea that heavy armor is somehow supposed to be automagicly better than light armor is fallacious. Heavy armor is what allows non-dex builds to have good AC. It's not supposed to be better.