The Down Under Defense Expert (DUDE) sent me an email asking me to read a voter's referendum they're considering in New Zealand. Feel free to read it yourself. I just wanted to expound a little bit on my answer. Here's what I wrote to the DUDE:
"I found it maddeningly complex. But then I'm used to the US system of winner-take-all (the NZ referendum calls the system First Past the Post (FPP)) and having minority party issues subsumed under the major parties. That said, I've heard/read good things about the Australian system. The others were variations on a theme that made little difference to me because I have little trust in the stability of governments with a multiplicity of parties.
"The advantage of a major-party system that incorporates minority views is that much of the assimilation and wheeling/dealing is done prior to the election. Otherwise, you end up with one small party can upset the entire governmental apple cart.
"I fail to see much advantage to the systems that encourage the minority parties, nor do I understand NZ's desire to change their system or the bewildering number of distinctions without a difference. If what you have works, don't fix what isn't broken."
There are three other systems: the existing NZ system (Mixed Member Proportional), the Irish system (Single Transferrable Vote), and the Japanese system (Supplementary Member). My observations are just that multi-party nations and governments seem more unstable than the U.S. The advantage of the multi-party system, if I had to guess it, is simply that it gives broader voice and more formal power to minority parties than exists in a single- or two-party system, as we have. If a smaller party (say, the Greens or Libertarians) had a strenuous objection to a particular law or course of action and their support was necessary to ensure a majority coalition, the government/parliament will be dissolved and new elections are called if no agreement can be reached. Most of these governments are designed along the British model, and nearly all of the major Western nations (incl. Israel and Japan) that use it have shakeups of that sort at one time or another.