That is the fear that is being used by some who are against AV to win the arguement, but as you and I both point out, it is a fiction. Extremists do not benefit from either AV or FPTP. And when they do benefit from a system of AV with party top up votes (London Assembly) they end up proving themselves to be idiots. (This is not the method proposed for Parliament).

The switch to AV is simply not about excluding one party or another, nor is it about having one party permenantly in government. It is about getting democracy working best for the largest number of people.