|Darksol the Painbringer|
Look at the ability wording closely; it says that it applies that bonus when worn, not holding or carrying. If it's not worn, the bonus isn't being applied to your AC. The final sentence says that even if you don't have it on your person, it still retains that power. In other words, if it's disarmed from you or you decide to unequip it, once you get in possession of it and wear it again, its bonuses reapply.
You are correct though, in that a Buckler is a shield and counts for this ability, as well as effects that ignore Shield Bonuses don't ignore the bonus this ability provides; i.e. Touch Attacks.
Also take note that it specifically calls out mechanics from the Channel Energy class feature, meaning you have to expend Channel Energy uses to power the ability. This makes the Extra Channel feat much more valuable, and that this ability overrides the mechanics one would try to use Quick Channel for.
Of course, a better question would be if a Disarm counts as being struck in combat; RAI, I'd think not, but you're still making an attack against the target's number, so it can go either way.