It's not always welcomed, exactly. Two days ago I was checking in with NW at MHT. Had my CO Platinum number in the system but wanted the miles on my NW WorldPerks account. The check-in agent accused me of trying to pull a "nice little scam," and took my Platinum card off behind closed doors for a couple of minutes to consult with colleagues about what to do to me.

I ended up taking the NW miles and no upgrades (I want out of OnePass THAT badly). I know others have reported NW agents helping CO elites out in this way, but you're basically at the mercy of the individual, and if they're feeling prosecutorial goodness knows what might happen.