I don't think you are using the armour system to its maximum potential. A single breastplate isn't meant to be the complete package at the moment. A "fully" clothed dwarf will be wearing 3 mail shirts and a breastplate and 6 cloaks on top of that.
Leather is best used for making cloaks. I think a step in the right direction would be to remove the shaped tag from leather as it would be quite possible to wear leather underneath a breastplate as I'm sure many a knight did.
http://dwarffortresswiki.org/index.php/DF2012:ArmorFor all your armor info.
Breast plates can get expensive and actually cover less area than leather armour. 1-2 mail shirts leather armour and low boots might be a more economical route for archers etc.
It should also be noted, that when untrained, dwarfs in armour preform badly in heavy armour. If the dwarf has little armour skills and is going up against real enemies (ie, your not trying to train armour at that time), they might be better off in leather due to the massive speed penalties of untrained dwarves in heavy armour.