Balance pendant will take the second highest defense level, and boost/decrease everything to the same level.
For example, if you have high normal defense, but super low elemental, but a moderate piercing defense, everything will even out to the same level as the piercing def.
But that'd be hard to impliment. Armour stats aren't easy to change or average, and this would cause a lot of 'balance' (dohoho) issues, amongst other things. Not to mention that if you wanted that, it'd be much better to use a mixed armour set.