I've been davening in a Nusach-Sfard (chasidic-style) shtible lately, because it has a morning minyan at a convenient time (7:45) without another mourner competing for leading the prayers.
I was chatting with the gabbai about my confusion between the Nusach Ashkenaz and Nusach Sfard texts, that Nusach Sfard has a lot of extra words stuck in. He commented that he once said the extra phrase in Kaddish, "Vayatzmach Purkanei Vikareiv Meshicheih" (his redemption will flower and his Messiah will draw near) at a Nusach Ashkenaz place, and they came down on him hard for it*.
At any rate, the gabbai was puzzled, who wouldn't want to invoke the Final Redemption during Kaddish?
I just came up with an answer. Look at the Kaddish. Aside from the phrase in question, it's all about God's existence, kingship, praiseworthiness, and relationship with Israel. Basically, expanding on "Baruch atah H' Elokeinu melech haolam." It's said by mourners largely as a defiant expression of their continuing belief in God in the face of ultimate tragedy, the destruction of one of the pillars of their personal worldview - one's parent.
So my return question is - while we're proclaiming the greatness of God, how is mentioning an earthly king appropriate?
* They may have feared he was a Lubavitcher, many of whom practice what I call "cultural imperialism" - they will insist on praying their nusach when leading prayers in a non-Lubavitch synagogue. Actually, I find this is mostly the position of ignorant lay Lubavitchers, while trained Lubavitch cantors who know the halacha will use the local nusach.