The reason they give makes sense, and I suspect the majority of people who buy it are doing so because it's a convertible Aston and not because they expect a V12, but if I'm buying a new Aston, it has to have that V12 because you're not likely to get all-out performance from the brand and the edge the Vantage may end up having due to the lighter V8 wouldn't be worth losing the V12.
In all it's a shame, because I do prefer the looks of the convertible, largely because it doesn't have those ugly B-pillars, but look at those seat backs!