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The LibDems got totally stitched up by the Tories in that coalition, such was there desperation to be in power.
A change of voting system should have been their absolute minimum criteria. To agree to let it go to a referendum was naive and showed their inexperience, and, you could argue, was the event that gave Cameron the idea to offer one on EU membership as it was a non-event with very low turnout.
I don't think that's what these systems generate in practice. It seems more common that unrelated issues are traded off against each other with some kind of tally in the background, leading to a disjointed policy programme.
The experience of the coalition post 2010 in the UK bears that out for me. We didn't get a broad Lib-Con compromise, we got the Con programme with some random goodies like the AV referendum thrown to the Lib Dems.