This excellent post discusses how AV reduces the number of safe seats.
There are many advantages of changing to AV and one of them is a reduction in the number of safe seats where MPs effectively have 'jobs for life' because the seat is unlikely to ever be close between the top two candidates.
I was going to write a full rebuttal of the NO2AV most recent leaflet which makes many dubious claims about AV, but Will Straw at Left Foot Forward has already done this, so take a look.
There are a few points I would like to add, in particular how AV actually works to reduce safe seats.
One of the claims of NO2AV is that AV will make no difference to safe seats because MPs there already get over 50% of the vote.
Over two thirds of seats are safe under the present system and very unlikely to change hands, yet only a third of MPs get over 50% of the vote in their constituencies. An MP can be 'safe' with just 40% of the vote because of the split of votes between other rival parties.
So already we can see that safe seats won with this percentage will be affected by AV. But more importantly than that, AV will change people's attitudes to their first preferences. People could now vote for their 'real' first choice however unlikely that candidate will win. So seats safe now, may not be under AV. This might explain why virtually every Tory and Labour MP in a safe seat is against change. Don't listen to them, of course turkeys will oppose xmas. We need to show them what we want, not what they want.
AV will roughly double the number of marginals and make all seats more competitive. We will not get another chance to change the system, reject AV and all change will be off the agenda for at least a generation.