The problem there is, +20% defence (I will assume damage reduction is meant) is not the same as +20% HP, then only having +10% healing on the extra HP makes them harder to heal. You could theoretically make them the same effective mitigation but, if they act the same to heal, then why bother? Plus, swapping on to tank stance is much more beneficial for the +20% mitigation as you get the benefit right away as opposed to having to heal the +HP tank to gain the benefit of the extra HP.

So, whilst you can make them statistically the same in regards to effective tanking, one has an advantage over the other. Would it cause you to take the PLD/GNB over the WAR/DRK? probably not, however, it could annoy a few healers having to heal them up as well as the MT for tank swaps. So, why make them different? All because of flavour?

I also fail to see how one is simplicity and one is homogenisation. They both do the same things (ignoring the downsides) and so it is effectively no different. You haven't 'simplified' anything, just made it look different.