Ever since I've been old enough to vote (and possibly before) I have believed that the First Past The Post (FPTP) system of voting for Parliament was deeply flawed. In days gone by when essentially 2 parties fought general elections, it was probably not too unjust, but the 1983 election showed the flaws. The SDP-Lib alliance got over a quarter of the votes, but 3.5% of the seats on offer. It's an election usually referred to as a Conservative landslide, but in fact the two main opposition parties got 11% more of the popular vote.
The problem is what you change to. The Jenkins Commission recommended "AV+" in 1998, but the promised referendum never came. It was a system that aimed for greater proportionality, but would have required massive constituency changes and a second type of MP. The more complex STV system is said to provide a better proportional result but again requires change to multi-member constituencies.
So Alternative Vote (AV) is the only practicable choice from FPTP. It isn't perfect (what system is?) but I think it has some features that commend it. In constituencies where one party is unlikely to win, under FPTP a party's supporters are faced with tactical voting or staying at home in despair. Under AV at least they can vote for the party they actually believe in as their first preference. And candidates would need an eye on the 2nd preferences, so negative campaigning would be less effective - which would be welcome.
Oddly, one of David Cameron's points against AV was its disproportionality. In fact it's an unknown what effect it would have, as we don't really know how people's voting would be shaped by a new system. It would probably exaggerate a landslide situation, but would certainly not permit government by a party which didn't have some consensus of popular support. If Cameron really wants a proportional system, I would welcome him introducing a bill to achieve just that, but I suspect his enthusiasm would fade pretty quickly!
One factor may be that the current Conservative administration thinks it would be disadvantaged by AV, but that's not necessarily true. It all depends on the political climate at the time. In the 1980s, 2nd preferences would almost certainly have gone Lab to Lib/SDP and vice versa, but we can't guarantee that now. The 2nd preferences of LD voters may go different ways depending on whether it's a LD-Lab or LD-Con fight (and you can work out the other permutations). It seems to me that no party has necessarily anything to fear from AV; however they will need to present their case and campaign in a different way.
The main objection to other voting systems is that they are indecisive and produce unstable government. I think the most recent general election proves that nothing is certain whatever the system, so why not go for one which might actually more accurately reflect the overall preferences of the people?