Unity makes the following assumptions;
That people will not quickly change their party allegiance under PR and that the Lib Dems will remain around 20% of the vote giving them the permanent kingmaker role.
That there is not a bigger 'leap of faith' or 'risk' that first-past-the-post will give the permanent kingmaker role to the Lib Dems.
I have argued that the opposite is in fact the case - it is first-past-the-post that maintains the Lib Dems as the repository for dissaffected Labour and Tory voters - as the Lib Dems give the only opportunity for protest voters to affect the result of elections under this perverse system. I have argued that ironically (because Lib Dems are big supporters of PR), if we want to see the back of the Lib Dems (and believe me I do) then PR is the way to do it. They will inevitably lose their support when people are free to vote how they choose. The Greens and Socialists will be quickly established at the expense of the Lib Dems (because at the end of the day the Lib Dems are a rag-tag of philosophies and ideologies and have no clear direction at all).
It is a curious argument of Unity's that the main parties would go for PR if only the Lib Dems (or any centrist third party) didn't exist - since both of the main parties have had plenty of opportunities when the centrist third party only got small percentages of the vote to implement PR but showed no interest whatsoever.
I have also argued that the key to getting PR is to persuade the Labour party while it is in government (a hung parliament reduces the chances of PR because it reduces the number of PR friendly Labour MPs)- and persuade intelligent people in the Labour party like Unity that PR is the answer to our problem of long periods in opposition. It might be easy to forget but Labour has spent the vast majority of its history in opposition under FPTP despite the majority of voters continually voting for progressive left-of-centre policies. This in effect is the 'Polly Toynbee argument' for PR and I believe it is a strong argument to use amongst tribalist Labour party members although the strongest argument overall is the moral case that PR is more democratic. When Labour is in opposition for long periods, PR seems very attractive indeed, but when in power we lose sight of what is in our long-term interests.
Centre-left parties (like Labour) tend to quickly establish themselves as the largest parties under PR and in conjunction with the Greens and Socialists spend the majority of their time in power - this is in contrast to plurality systems like FPTP. The explanation for this is here, but put simply - the left-vote tends to be more split between parties and this is punished under FPTP but not under PR.
I used the example of New Zealand to demonstrate what happens when a Westminster electoral system is changed to PR. The centrist third party (Alliance) which got around 20% of the vote under FPTP fell away under PR and Labour (in conjunction with Greens and socialists) have governed since 1999 winning three elections and increasing their share of the vote each time (compare that to the 4.5m votes Labour have lost under FPTP). Turnout was 81% and the largest increases in turnout have been in deprived inner city areas (who tend to vote Labour).
If Unity really does fear the Lib Dems being permanent kingmakers then he should take heed of what has happened in Canada. They have had 8 hung parliaments in the last 50 years under FPTP and the third party has been the power broker. Now the Lib Dems have established themselves with over 60 seats we could be heading the same way. There is a big difference between coalition government under PR and hung parliaments under FPTP. Under FPTP you are guaranteed that the centrist third party will be power brokers. Some people get so frustated by FPTP that they even vote Tory for 'a change' because they know they only have two choices of government. PR will finally allow people to vote for policies they really want.
The battle for democracy continues. It took a long time for all adults to get the vote, now we need to fight for 'equal votes'. The people know something is not right with our democracy but enough people haven't woken up to the fact that PR is the next step to improve things. Change has to come from one of the main parties and the Labour party are the only main party to have shown any inclination towards PR. The Labour party in government is probably the only way we are going to get that change. Labour should not fear democracy because what is best for the people is best for our party.