That was pretty much my thought, but I think the case given in the video is a bit more than that - it's not just that it opens the door to negative campaigning (Edited to add: and hard-ball negotiating, which was what I was thinking about when I first wrote), but that it could damage the perception of commitment to a currency union, and in turn reduce the actual likelihood of agreement in a tough negotiation if that commitment's in doubt.

We may not be able to call it pre-negotiation exactly, since there's no talking, but it does seem to be a stance taken with successful negotiation very much in mind, at some risk to making the public, political case that'll establish the need for negotiation.

It must be quite a difficult stance for a politician to take, despite that political argument about the muddied waters in its favour too, and I'm glad that sort of thinking is going on. I think they should make more of a public case of that argument than they have, though.