Well, 15 attack almost guarantees successful strikes. 19 damage essentially guarantees a 1 hit kill on lighter troops, and guarantees damage to heavily armored forces. 13 attack is still pretty good, but with 16 damage you will be far less efficient about making your kills. Plus the 2 prot jumps them from medium infantry, to heavy infantry, drastically reducing the effectiveness of the enemy, as well as the jump from 9 to 11 HP making them less likely to die. And a measly 1 encumbrance difference should not be a factor, because these guys kill other infantry so much more efficiently, they shorten battles when they get in close - as well as providing a very beefy line to screen your archers so that they can continue exercising those long bows.
Again, I think samurai archers are awesome, but I just can't agree that they are shock troops, and shock infantry is a very potent tool that many nations lack in their arsenal but that Jomon has in spades.