Your disposition is wrong. You might just as well say you don't get the AC from a Defending weapon if it's in your offhand and you also use the Two-Weapon Defense feat.
It is?
What you advised is incorrect as the AC bonuses come from different sources. And this does not apply to my question which was: Why do class AC boosts stack?
There are plenty of builds on this and other boards that use Swordsage and Monk to AC to elevate the characters AC to great heights. Another is Kung-Fu Genius and Iaijutsu Master. Another Deepwarden and Fist of the Forest.
These are taking AC boost from the same source. Wisdom. In the case of Swordsage and Monk, this bonus theoretically would come down to combat training to avoid getting hit. Hence you are wise enough to get out of the way.
So what I'm looking for is a reason that they would stack.
One is a replacement, the other is a bonus. They stack just fine.
Agreed in the case of FoTF and Deepwarden. Not in the other cases.
They do not come from the same source. They come from separate sources, they just use the same mechanic to determine the benefit of the separate abilities.
No they come from different class abilities which are doing the same thing. It is like having two dex bonuses added to your AC.
The source of the AC bonus is an Ability Score. You can't have Dex to AC twice. Why can you have others?