The winner should be the candidate that comes first, but under AV the candidate who comes second or third can actually be elected. That’s why it is used by just three countries in the world – Fiji, Australia and Papua New Guinea. Voters should decide who the best candidate is, not the voting system. We can't afford to let the politicians off the hook by introducing a loser's charter.We have no idea why it's only used by just 3 countries in the world. Perhaps, like the UK, a lot of countries with an FPTP system have 2 parties who would like to keep it that way and never get the opportunity for a referendum. Some have chosen STV or PR systems instead. I personally think STV is better or some form of multi-seat representation.
As for the rest of it, the point of AV is that it's actually a more accurate way of choosing who the best candidate is because it allows for more expression over choice. It allows us to see what people's general leanings are, what sort of philosophy they sign up to, which means that two candidates with similar philosophies don't see votes getting split and another candidate winning.
One of the stupid parts of the FPTP system is that one of the best ways to spend a few grand in politics is to fund a party with a philosophy similar to your opponent and hope that it then splits away your opponent's vote, letting your favourite candidate win. Think about that, and tell me it actually makes any sense.