The Alternative Vote makes it hard for extreme parties to win seats, because candidates need broad support across communities. (politics.co.uk)
The «no» campaign has claimed AV will cost # 250 million, increase the number of spoiled ballots and benefit extreme parties. (politics.co.uk)
As the electorate becomes more and more fractured, small extremist parties might not actually grow in size (the voter base for extreme views remains the same), but the need to get that last one or two votes to build a majority coalition becomes greater and greater, making the negotiating position of extreme parties much greater, and allowing them to force their platforms onto the coalition in exchange for their vote. (worldbuilding.stackexchange.com)