First Past the Post is the cornerstone of the brokenness of our politics. It has the power to subvert all your campaigning efforts for a good cause, making that cause weaker instead of stronger.
Support the environment? Vote Green where the Lib Dems could win and you do more harm than good.
Support Euro-nihilism? Vote UKIP where the Conservatives could win and you have the opposite effect.
Support socialism? Vote Socialist Labour, where Labour could win, and, well maybe the pattern breaks down there.
Want to stand as an independent? The problem is that whichever party it is you consider least bad, you will take more votes from them than from the others. So unless you can be very confident of winning, you will only make things worse.
AV fixes all this. It lets you stand and campaign for what you believe in without damaging the causes you support. It lets you vote for what you believe in without having to second guess what the result will be and support the lesser evil.
Sure, it's not proportional. That stinks. But proportionality is not the only important feature of an electoral system. If it were we would support list systems rather than STV. And this is the worst time of all to hold a referendum on a proportional system, when the BNP have just won seats.
If Brown and his few friends plump for AV, we will probably get it. We could even have it for the next general election - it is the only reform that won't require years of boundary commission work. And it may be a stepping stone to STV or AV+, but not towards party lists.
If they plump for PR, the whole thing will be quietly dropped, "after considered reflection", the next time Griffin gets some publicity. Or, even worse, we have a referendum on it after weeks of the Tory press giving publicity to the BNP at every opportunity in order to get a "no" vote. It would then be off the agenda for decades. This is the single worst time for a referendum on PR. Give it a few years when a couple more nazi MEPs won't have brought the world to an end.
See also: Anthony Hook, Mark Reckons, Himmelgarten Cafe, Jock Coats, Sunder Katwala, and Duncan Borrowman (already linked)