I know that changing the electoral system doesn't solve everything. For example, educating the electorate is just as important, because with a biased media, any electoral system change is of limited use. Italy and Russia are extreme examples of this with Belusconi and Putin's control of the media!
But apart from that priviso, I've racked my brains to think of reasons to keep 'first past the post' FPTP and I'm struggling to think of anything. I don't think it even leads to decisive government!
Look at all the long term big decisions that have been avoided on our infrastructure, the economy, transport, the environment! All issues where Germany has a much better post war record. Look at how effective coalition rule has been in Scotland and Wales and even London.
Under FPTP, parties have to always fudge the issues to win the marginal seat floating voter rather than saying what they think and building support gradually.
FPTP is not even the best non-PR system! The Alternative Vote (AV) is much better because it at least takes account of voter preferences.
The only thing I can say in FPTP's defence is that, because it has been in place for so long, some of the electorate have learned how to get the most out of a bad system by voting tactically etc.
Any eventual change will be painful in the short term as the electorate and parties get used to a new system. It will take one or two elections to bed in. This I think is the major obstacle and why AV might be a useful stepping stone towards PR. Although an Additional Member System(AMS) is my preferred solution overall.