Though Haruhi's power is indeed fantastic, we understand how it works to a large extent. If not for our reasons to observe Haruhi, than the existence and presence of myself, Nagato and Asahina can be explained by Haruhi's desire for us to exist.
In short, we understand why we have been asked (sometimes by force) to join the SOS Brigade. The one we do not understand is Kyon. What reason did Haruhi have to ask a normal human with no special abilities to join the Brigade?
Though this remains a mystery, I believe that Kyon is the key. The key can have no significance itself, but will activate something of significance in the future. In other words, Haruhi chose him for a reason. We simply don't understand why that is yet.
As for his awareness of our powers, I believe it to be necessary in regulating Haruhi. I do not know why Nagato and Asahina revealed their identities to him, I would assume they believe he can be of some assistance to them, but he is very convenient to me personally, as he can have a significant impact on Haruhi's mood and actions. If he can prevent Haruhi from becoming upset, and he can, then less closed spaces will open, and I have less celestials to hunt. The risk that the earth will be destroyed is decreased. In this way, Kyon's ability is beyond any of ours for whatever reason.