It encourages the participation of minor parties//candidates, and of voters who might otherwise say "a plague on all your houses" as there is no candidate that remotely reflects their views. Of course in a 5 seat constituency, it is also quite possible for a candidate with less than 10% of first preferences to get elected provided they can pull in enough second and lower preference votes as other candidates are eliminated.
Extremist parties parties do less well as they get lesser transfers of lower preference votes (many people will never give a preference to e.g. Sinn Fein or Fianna Fail) but an outstanding candidate, e.g. Joe Higgins of the tiny Socialist Party can get elected.
Of course such a system is hopelessly complicated for the Brits and really only suitable for the Irish... Frank's Home Page and Diary Index
If the first preferences are also used as the vote for a party list in a PR component (either mixed representation or House of Lords), there's an even stronger incentive to keep your LOTE vote as your 2 vote.
And its a lot easier to announce 2nd Pref, Instant Run-off votes on the night than Full Preference, Alternative Vote. I've been accused of being a Marxist, yet while Harpo's my favourite, it's Groucho I'm always quoting. Odd, that.
I have a particular hatred for STV (complicated, non-transparent, open to manipulation by the major parties), largely based on Australia, where it's an unmitigated failure in terms of boosting representivity. But that's perhaps because Australians are particularly (bites tongue and does not type "stupid") lazy about politics, they expect the major parties to be cynical and corrupt, and in their majority they play along with them. This is worsened by the fact that voting is compulsory, so that large numbers of people who really don't give a toss turn up and tick the box for a party's rort ticket.
The Irish counter-example is fascinating. It is rightly acknowledged that people of faith have no monopoly of virtue - Queen Elizabeth II
