Voting systems: all things to everybody

Or no things for anybody.

According to William Hague,

AV is unfair. With First Past the Post, everybody gets one vote. But under AV, supporters of extreme parties like the BNP would get their vote counted many times, while other people's vote would only be counted once.

So lets get this straight. According to the No campaign, AV is good for the BNP (boo hiss vote NO). So what about the Yes2AV campaign?

Well according to Yes2AV partisans FPTP is good for the BNP,

4. Shutting down Extremism
Extremists can get in by the back door under FPTP. We‟ve already seen BNP candidates elected to our town halls. And in each case, they have scraped in with minority support. AV doesn't let candidates slip in with just a minority of the votes. Winners need the goodwill of the majority. For extremists AV is a brick wall.

This of course leaves me a little confused. Because what goes for the BNP must go for the Greens, UKIP and any other of the smaller parties.

If as supporters of both sides claim, their own system is the better system for our democracy, how can both sides boast how their systems will keep out the smaller parties?