While First Past the Post can deny due recognition to minority parties, it is also, paradoxically, far more vulnerable to extremist manipulation than AV.
Minor party supporters who vote with their conscience, waste their vote. If they vote tactically, they deny their preferred candidate support. People usually vote with their conscience. While this is highly honourable, it can let the extremists in by the back door.
The most recent example of this was the election of George Galloway for the seat of Bethnal Green and Bow in 2005. Galloway represented the so-called Respect Party. If you have any doubts about the extreme nature of Respect, which is essentially an umbrella organisation for various extreme left wing and Islamic groups, have a look at WikiPedia Page Here.
The Bethnal Green and Bow constituency has a large proportion of Bangladeshi immigrant voters. It appears they voted en-masse for Galloway due to the Islamic identity of Respect and also because the incumbent Labour MP (Oona King) was a Black woman of Jewish-Afro-Caribbean ancestry.
Even so, Galloway only secured 35% of the vote. But he won with a majority of 823 over Oona King.
If this had been a AV election Galloway would have had to make it to 50%. I find it difficult to believe that if the Liberal and Conservative vote had been redistributed with AV that Galloway would have won.
The end result was that the 65% of good people of Bethnal Green and Bow ended up being represented by an extremist party, rather than the capable Oona King.
Pro FPTP supporters often moan about how with AV, the second (or even third) choice on the first ballot may potentially win. Well, I reckon the majority of the people of Bethnal would have been overjoyed to see such a "travesty" in 2005.
Interestingly in the 2010, Labour regained the seat - with an Asian candidate who replaced Oona King. Many have commented on how Labour cynically sold out to the inherent racism of the Bangladeshi community in Bethnal simply to regain the seat. With FPTP they had to.
I understand Oona King was promoted to the House of Lords (Baroness King of Bow) but I understand, from several sources from she was a considerable loss to mainstream British politics
Below is the Bethnal election result for 2010. You will see the emergence of another extremist party - the BNP. They may have only gained gained 2.5% of the vote, but it is a foothold. People who are disenfranchised by FPTP are easy pickings for extremists. Perhaps what the BNP hope for is that one day, due to the unfairness of FPTP they may, on a fluke, secure 35% of the vote and so gain the seat - just like George Galloway.
Maybe that's the reason the BNP among others, are so keen to see AV voted down.
We must replace FPTP. It is a decrepit system ill suited to a modern democracy. AV is far from perfect but it stands head and shoulders above FPTP.