Just a quick one. AV does not give disproportionate weight to unpopular parties. What it does is ensure that of everyone left standing in the race in each round, people have an opportunity to say which they prefer.
Round 1: Candidate A, B, C and D get 40%, 20%, 25% and 15% of the vote respectively. The least popular candidate, D, doesn't get a disproportionate boost, he gets eliminated.
Round 2: Candidate A, B, and C now get 40%, 33% and 27% respectively. What people who don't understand AV, or are wilfully misrepresenting it, are trying to say here is that Candidate B gets an unfair weighting. It's just not true, Candidate C drops out here because those people that voted D have the fair opportunity to say they would rather candidate B got their votes rather than candidate C if their most preferred candidate is eliminated.
Round 3: We're left with Candidate A and B, who on balance had the most support of everyone to be put in to this final showdown. The final result is 45%, 55% respectively. A loses. Again, this isn't because of disproportionate weighting but a fair opportunity given to people to say what they would prefer if the situation was such that their first and second preferences were no longer in the competition.
There is nothing more fair than coming to this kind of compromise in single member constituencies, where everyone at each stage gets to have their beliefs tallied so that the most popular candidate of those remaining in the vote, at least one of which will be due to the initial high popularity within the largest minorities of the constituency.
People who voted for Candidate A in this scenario gave their candidate TREMENDOUS weighting, they kept him in the competition all the way until the final round with no fear of elimination. The fact Candidate A then didn't do enough to ensure he had broader support in the constituency if people were ultimately asked to choose between him and candidate B is a seperate, and more political, issue.
It is, quite simply, a myth to say that in this above scenario that Candidate B, despite coming 3rd in the FPTP style vote, is a "less popular" candidate. How can they be less popular if, when the voting options narrow to A, B and C, that candidate then picks up enough votes to not be the last in terms of percentage share?
It's a fallacy to claim that AV gives disproportionate weighting to the "unpopular", what it does is more accurately show who is really popular or unpopular on balance as the voting plays out. In this case A was simply popular with a significant minority of the constituency, but when it boiled down to it was not as popular as B in a straight head-to-head.
Under FPTP we simply don't know this level of information. Is the winner of FPTP on 40% the most popular? He could be, in one sense he is, but in another sense he may not be. Without information rich voting systems like AV, we just don't know for sure who is popular or not
For more on why compromise is good, and why trying to apply weightings to each person's vote is a flawed concept, please see my last post on this subject.