I see two coherent arguments for AV:
1) Some favour it because they believe it to be in their own partisan advantage.
2) Others favour it because it tends to encourage centrist coalition-based politics.
I see no coherent merit whatever in the other case commonly advanced: that it means that all candidates command more than 50% support. Apart from being plain false (if you get 35% of first preferences and 50% of second preferences, you are supported by 35% - AV or no AV), I also see no value whatever in the majoritarian fetish. But I've argued against that before, and will leave it for now.
Here, instead, I wish briefly to counter the other two arguments.
First, making long-lasting constitutional decisions on the basis of personal partisan advantage is surely amongst the worst kinds of politics. It is arrogant, as if your own partisan advantage were of long-lasting import. It vests power, and encourages decadence and corruption. Once you set off down this path, you will find many future opportunities to manipulate the constitution in your own favour, and the Rubicon will have been crossed. Don't go there.
Slightly more complicated is the argument for centrism. Some people think that in politics there are those of centrist, mainstream, sensible political views and then various degrees of extremist. They see merit in political systems that in the main lead to centrist governments, preferably based around compromises between coalition partners so that the centrism can be protected by the continuous threat of collapse of the coalition if extremist policies are adopted.
In contrast, others see politics as fundamentally about choices. Some are technical choices, where there will be straightforwardly correct or incorrect answers (though these may not be perfectly obvious in advance). But most will be moral, philosophical, or politco-existential choices.
I favour the choices view of politics. I have argued previously that one of the key values of democracy is that it enables voters to participate in high-level moral, philosophical and politico-existential choices. They lack the competence, time, or inclination to participate meaningfully in technical choices. So the idea that technical mainstream common-sense policy-making is protected by centrist- or coalition-favouring constitutions is misconceived. Furthermore, as I have argued repeatedly before, the ruling oligarchies are not seeking to place all options before the Electorate, as if it ruled itself. Only limited choices need to be offered to the voters, but when those options are set out, they should express clearly distinct positions and facilitate voter choice.
Since virtually the only worthwhile argument for AV is that it encourages (post-electoral) coalition (and hence centrist) politics, if, like me, you believe that elections should be about choices placed before the Electorate, you will see that AV is a system with virtually no merit at all.
