Sinhababu pointed out that Americans who complain about the lack of choice involved in the two-party system is actually an illusion; there's plenty of choice available to voters who participate in primary elections. Yglesias jumped in:
Perhaps nobody cares, but I think Neil Sinhababu is right to say that America’s system of fairly wide-open primary elections serves many of the same kinds of functions as the existence of multiple political parties in proportional parliamentary systems.It's true that American primary elections don't really serve as rallying point for ideological factions, certainly not beyond a moderates vs. extremists framework. But nomination contests really do serve to give party activists and aligned interest groups an opportunity to introduce new issues into the party platform, and to enforce control of issue positions over politicians. I've talked about a couple cases of that in this year's midterm primaries on the Democratic side: Democratic candidates are likely to strongly support adding the public option to ACA, and filibuster reform in the Senate.
The interesting thing is that they’re not all that often actually used in this way...In the United States it would be possible in principle for self-identified libertarians to attempt an entryist strategy where they get deeply involved in organizing for primary candidates. But in practice they don’t. When primaries with ideological content happen, they tend to pretty generically pit a “more conservative/liberal” candidate against a “more moderate” alternative.
Of course, a system doesn't need the direct primary to be open to voter and activist input, and I'm pretty ambivalent about whether it's a good way to perform that function. It's nice, I think, that voters (that is, voters-as-just-voters, who don't otherwise get involved or want to get involved) have an opportunity to register their opinions, but we know that it's the people who get more involved who really affect nominations (both in the sense of who wins and the sense of what it is they stand for), and a caucus/convention system is, in some ways, a much cleaner way for those people to get involved. The important principles involved for a democratic system are (1) that parties, however broadly defined, should be able to control their own nominations; and, (2) those parties should be permeable and relatively non-hierarchical.
What I do think is very good in the American system is the relatively decentralized system of nominations. That helps to make the system more permeable and less hierarchical. Basically, it turns out not to be very difficult to get involved in party politics at the Congressional district level and, over time, really affect nominations. One of the reasons that I like first-past-the-post elections is that single districts, which nominate their own candidates with relatively little outside influence, are good for encouraging that kind of involvement. To the extent that national parties slate candidates in Congressional elections, I think that we lose a bit of democracy from the system.