I dunno. While I think it's a good suggestion, I like how Aegis Boon operates now.

It basically nullifies a melee attack and returns HP. That's great. It's even more fantastic when it completely negates a TP attack, which you wouldn't really be able to do if it was a type of stance.

I prefer being able to completely nullify a TP move with 100% chance over having a slightly better block rate for a minor HP return upon a block.