In FAQ1.6, we know that legends can defend capital zones if a card effect allows them to do so. (I think we'll see this kind of card effect in coming future... )

Suppose that a legend is allowed to defend any of your zones, when it defends my kingdom zone, how do I count the number of power icons on the legend for assigned damage? Use the kingdom power icons of the legend? or "A Legend’s power is considered to be equal to the number of power icons it has in its weakest zone"?

Thanks for your reply...

I guess actually, it's just impossible to answer your question. You can just make supposition. This will depend of the effect giving the ability to defend.

If a card effect references the power of a Legend, then you take the power of the weakest zone, but for combat, I believe the legend always uses the power in the zone being attacked, or the power in its battlefield if it is doing the attacking. The FAQ/rulebook should be amended to make this more clear though. I think all it currently says is "If the legend is attacked, it contributes the power it has in that zone (as damage) to the battle."

Edit: This is all assuming that you have an effect that says "target unit or legend can defend any zone".