I upvoted this because I agree that the UK and Australia have more than two important parties, but I believe that the Australian voting system is MUCH better than the US and UK systems (which also supports your main point in a way).
Having seen MMP play out on the other side of the Tasman, it's pretty clear that MMP produces saner outcomes.
My personal opinion is that is MMP forces coalitions more often. The upshot of which is that everyone is forced to be more willing to communicate and compromise and take on saner positions, because the opposition one term may turn out to be the maker of the governing party the next.