gotta admit u're right.Not really. The simple fact remains that if Itachi had a Justu that could kill anyone, including Pain and madara, he would have done so; as they were the biggest threat to the two most important things in his life, Konoha and Sasuke.
Every Jutsu has a counter and Itachi was no fool, there was a reason he did not --could not-- attack either of them.
I have some info from the data book about Pain below, clearly this info did not turn out to be 100% accurate, yet it was there. You need to realize that for a story to be exciting, every superman must have his Kryptonite, every jutsu we have ever seen has had a counter, Susanno is no different. If Sussano is unstoppable then Pain is apparently an unkillable god. Also note that each data book only reveals as far as the manga has gone and an immortal in the data book one week can be dead in the manga the next.
~~Info on Pain from Naruto Databook 2~~
-People will know pain. Nations will know pain. The world will know pain!!
-The shinobi who bears the eyes of the Six Paths. The God born of conflict.<<< Pain is a god?
Page 3
-His Divine Will is like a sword, for grand is His desire...
-The deeds of God will taint even the darkest of nights in a bloody red.<<<God again?
Page 3
-Invincibility, as befits a God. And a mountain of mysteries behind it.<<<Pain is Invincible? Didn't they say the same thing about Susanno..hmm?
[Unkillable]
No matter the kind of jutsu employed, a God cannot be killed. Unkillable, thus invincible... The one Path who made it into the kekkai space suspended all activity, but it is unclear if he died or not.<<<This sounds familiar...Unkillable in the same way as Susanno..hmm?
Take from it what you will, but the point is obvious. Sorry to dissapoint all the Itachi fans. =(
but here again Susanoo won't have any weak points if we don't see it again in manga.untill then it's superoverpowered jutsu