Around this time every 4 years, we go through a period where people complain about the 2 party system in the USA. The complaints come from both the left and the right and probably reflect the realization that neither of the main candidates perfectly reflect their views. To me, this has always been a facile complaint that approaches but doesn't quite hit the levels of annoyance that "If Candidate X wins, I'm moving to another country" comments.
My issue is not with the idea of the US moving towards a multiparty system. I think that there are strengths and weaknesses associated with it and could learn to adapt. My issue is that I don't think the people who make these comments have fully investigated how multiparty systems work. It's true that having more options mean that you are more likely to find a candidate that is a match for your (most important) political views. At first blush, this seems to be a better form of representational governance. However, there are costs that go along with this. In most systems, coalition governments must be formed amongst parties that sometimes do not have broad a policy base as the major parties here. These coalition governments sometimes fall apart triggering new elections. Whereas the US system forces you to compromise by picking a candidate that may not be a great fit, you can generally expect a fair degree of action and good responsiveness (perhaps until recently) once the elections are held. In the multiparty system, it's more difficult to bring majorities together to get things passed because of the wider range of views that the candidates and parties can and do have.
There are ways to work around this, but they don't always get used. Instead, I want to focus on a comparison between a multiparty system and the current US system. Something of great interest to me in multiparty elections is how a candidate chooses to position themselves. In the US, there's the general and accepted trend of a candidate moving towards the base during the primary season and then shifting towards the center for the general election. The degree of shift varies, of course, but I think that everybody at least repackages the portrayal of their views. However, in a multiparty election, this is not always the best idea or easiest path to power. For example, take France's most recent elections. They have a first round of voting amongst numerous parties/candidates and then, if nobody achieves 50%, then the two leading candidates runoff against each other. This sounds similar enough to our primary system. In this past election, the two parties that made it to the runoff were a center-left party, the Socialists, and a center-right party, Union for a Popular Movement. Interestingly, they did not move towards the center. They moved towards the extremes. The center-right party's incumbent candidate cynically focused on the threat posed by Arab immigrants in an appeal to the far right. The center-left party's candidate focused his campaign on being even more harsh on the wealthy than he had previously been. His party had proposed a 75% tax on the upper class, but the party to the left of them proposed a 100% tax on the upper class (basically, all income over ~500K). So while you may get the benefit of having a candidate that is a better fit, you run the risk that the party in power is much farther from your views or is even more extreme. Moreover, it is difficult to see how this system produces less vitriol or arguing like so many in the US seem to be yearning for.
Of course, it could be argued that we've already seen a shift in the practice of candidates moving towards the center during the general election. Karl Rove is widely credited with being the motivating force behind the Republican party's decision from 2000-2006 to not try as hard to reach for the center. It was almost certainly based in sound political data and theory. With a consistently decreasing number of people who are undecided to reach for, it's certainly possible to reach a point where it makes more sense to motivate and drive your base to vote in greater numbers to make up for lost undecided voters. (The dirty secret of undecided voters, of course, is that most of them lean one way early on but do not make their final decision until late.)
The common criticism of this is that makes the country more polarized. I agree with that criticism, but don't see how a multiparty system fixes that. If anything, it makes it worse.