But having said that, I think this could add an interesting level of strategy, but I wonder... at what point would you rather block than attack?
When the enemy attacks kill you before you can kill the enemy? Isn't that kind of obvious? You have two people in the back row who can still DPS while shields up front tank.
Just to add to peoples thinking on this...you can swap the character positions in the party at will, during combat as well. Guys in the front taking too much dmg in a battle, throw your back row party members up front to take more dmg off the others.
I don't see the need to throw in a Block mechanic as things currently are. I'd rather Move than skip an attack to use Shield Block.