I'm going to vote yes, because as has been pointed out it should considerably reduce the safeness of many safe seats, thus re-enfranchising voters in said safe seats who don't support the previously "home-and-dry" candidate, and therefore might in the longer term increase turn-out and make our democracy a bit more genuinely democratic. In essence I'm with Iain: I think FPTP only really makes sense in a two-party system. And I think the horse-trading goes on now, only it goes on in terms of which candidates parties put up in which seats, and is therefore far less visible to the electorate at large than horse-trading post hoc on the basis of votes actually cast.

ETA: Damn, sorry, for someone who's been writing drabbles all month that was a shocking failure to keep to word-limit (114!)