Appearance of statesmanship, Show the country that coalition government works and does not lead to sky falling in
Democratic legitimacy, Present this as "the most votes" result, Get some policy concessions
option to bring back the PR question after withdrawal of support, in better economic climate
Anger of progressives
Low chance of PR
No guarantee that Tories won't withdraw support and force a new FPTP election as soon as they get a honeymoon bounce, Not sure about this one, Tim. In a coalition, if things are going well, can the Tories really hope that they'll be the ones getting the credit?
Without significant concessions on PR, this looks like a bad deal. Cameron won't get party backing for PR, so don't expect this outcome
Lib Dems, Make sure there are terms in the deal that include "designed to fail" clauses so that there is a legitimate excuse to withdraw support and get PR vote
Progressives, Need to demonstrate their anger, eg through organised demos
Conservatives, As AB points out to me, the big question for Cam is whether to offer a PR referendum or not ... I think this is too big a risk for the Tory party and they will prefer to enter the PR question on the back of a broken coalition deal with LDs, PR is especially destructive to the Tory party organisation because of its local association structure., AB counters: "But it's hard for them (ie Tories) to refuse to offer it to the people", TCP - "I am not so sure. Tories know that a PR offer from Lab is not worth anything much"
High chance of PR
Close on much policy
Even a referendum on PR could be seen as an achievement, creating a big national debate, even if not successful this time.
anger of anti-lab voters
hostage to minority parties
danger of exacerbation of economic woes and being blamed for them, financial markets, whether right or wrong, will punish a horse-trading government with higher borrowing costs
The anger of the anti-lab voters and LD party members is probably not a big problem. Quite a few progressives angry with Lab will quite like to see them in coalition, while the deeply anti-lab are lost to the LD's anyway. The issue of hostage to minority parties could be very painful and could undermine the argument for PR; imagine a referendum for PR in which DUP are making what look like unreasonable demands and getting their way ... Very damaging to PR cause. But maybe this option offers the possibility of voting reform WITHOUT a referendum on it? This option looks good only if the reward is PR, but the risks to PR today seem high
Progressives, Claim that the "people chose to reject the voting system in THIS election"
Smaller minorities, Expect them to sound very reasonable, but don't trust them
Conservatives, Need to stress the anti-labour sentiment; this is the "Change" motif in their PR, Blame economic woes on fear of coalition
Option to bring down Tories when they're weaker, Danger of a honeymoon offset by the political costs of austerity. Lib Dems can control the calendar.
Missing the historical moment, Tory honeymoon and new FPTP election, Tories put the country on crisis footing and unite behind a leader
Failing to demonstrate that coalition politics is a good politics (hat tip Peter Johnson)
The option of a "better moment" must be discounted by a hefty probability that the country rallies behind a leader in a time of (economic) crisis. This option seems too risky.
PJ's point is a clincher, I think: how can you argue for PR when you've refused coalition politics?