The reason for the failure to appreciate this, I think, is that when a theory is being implemented properly, you just don't really notice when it avoids potential problems. I mean, if I had cancer or leprosy or some other such malady, I'd probably notice that I suffer from them. But how often do I notice my not-having-cancer-or-leprosy condition? Pretty much never.
Now a pastor could never actually pull this little stunt off, but I think it would be a fun experiment for a minister who holds to the two-kingdoms model to subtly depart from it for a few months without telling anyone. Preach the cross and empty tomb a little less, and preach about social issues a little more. This would work especially well if said pastor's views on cultural matters were not exactly the "correct" ones that the congregation would agree with.
Can you imagine it?
"OK, this Sunday at Random Presbyterian we're beginning a new 16-week series of sermons entitled 'The Christian Worldview.' In the weeks to come we'll be considering from Scripture such exciting topics as 'War ON Terror or War OF Terror?', 'Love Thy Neighbor: A Defense of Universal Healthcare,' and my favorite, 'Nike: Shoe Company or Babylon Mother of Harlots?'"
As silly as this sounds, it demonstrates precisely what the doctrine of the two kingdoms protects the church-goer from. In other words, it's all fine and dandy to beg your minister to speak out against cultural evils, but what happens when he takes your advice and rails against issues you support and defends the ones you decry?
We have six days out of every seven to "do earth," so on Sunday you'll just have to pardon my heavenly-mindedness and forgive me if I want to focus on things like Word, water, and wine. If this is "irrelevance," sue me, because the day I become "relevant" as defined by this age is the day I quit pastoring and take up professional punditry.