Martial Arts already has knockback on Crane Kick and knockdown on Dragon's Tail. As you mention, Cobra Strike has a powerful stun. Stunned/Knocked down/back NPCs are not going to hit you, so this is a sort of pseudo-defense bonus to the set.
You can further these secondary effects through enhancements or add in IO procs to further add flavor to the set.
Adding a +Def or -ToHit to the set is not necessary IMO. Roll a MA/SD or MA/SR scrapper and you'll have plenty of defense. 
Shard Warrior - 50 MA/Regen/BM Scrapper
Founding Member and Leader : Shadow-Force
Co-Leader: Council of Heroes
"Whatever evils come this way... we will be there to stop them."
I was thinking of Martial Arts and I was thinking about how it's about damaging but it's also about not letting someone hit you.
I suggest that a +defense or -tohit be added to one of the powers.
If +defense...make it the same value as Parry.
If -tohit...make it the same value as Touch of Fear.
I'm thinking the best power to add this to would be Cobra Strike.
Sure it gives the Stun...but its damage is extremely low.
What are your thoughts?