Good call gos_Jim, and yes we did it primarily for balance. However, there is a little "flavor/theme" to the decision:
The Scimitar is almost like a ranged attack, in that it is floating off by itself, making an attack on its own, and that is why it does not trigger a damage barrier or counterstrike.
Ranged attacks do not remove a guard marker because they lack creature presence and engagement. When a creature makes a melee attack, its body interposes the other creature's direction of movement, and the size/presence and active continuous engagement is what will remove a guard.
It is the creature presence, more so than the attack itself, that takes guards out. A crossbow bolt hurts, but won't deter a guard (unless it stuns him or kills him). But a big Orc standing in front of the guard ties him up while the other Orcs go around.
Yes, you could say the Scimitar has some presence to it of course, but its not quite the same as a creature. We looked at as the Sword flies out to strike and then comes back again, and there is no active engagement.
Mage Wars has been challenging to design in that we try to have a realistic simulation while also maintaining balance.