Depending who the friend is, or how desperate I am for some socialisation, I'll either answer "no, thanks, I just can't make it that night" (I have kids, no-one ever argues with that one), or "sure I'll come for the socialising, but I'll need to leave my credit card at home I'm afraid". The two times I've used the second line, the hostess has answered "no problem! I'm only in it for the free gift!"
None of my friends in this town are doing it, though. It was rife in our last city. I wonder what the difference was.