Keep activation time short then, hit it and it's only up for 1/4 sec or something before you drop to a ready stance again. Blocking could also slow down movement speed (which aiming arrows should do too btw), block all you want, but you'll quickly become surrounded and attacked from the flanks or behind where you can't block.
I can see your point with wanting to keep combat fast, but the unreliability of the auto-blocking vs. melee attacks can get very frustrating.