In a less ideological point, I'd argue that a political monoculture is part of the issue. It's hard to hold local politicians accountable if there's no real chance for repercussions for being corrupt or just doing a poor job.
Anyway, in my experience the biggest issue with primary votes is that the vast majority of the population doesn't care, doesn't pay attention, and most don't even bother voting. Check out the turnout for most primaries (and California's "jungle primary" doesn't seem to have helped with turnout[1]), or ask the people around you about the positions between the two candidates in some of the recent local primaries.
[1] http://www.latimes.com/opinion/op-ed/la-oe-meyerson-californ...
California does that for most formerly-partisan offices (excluding Presidential electors), not just Senators.
> In places with political party monocultures it gives alternative views within a party a shot at winning a general election.
Specifically, in urban areas that tend to be supermajority Democrats (there's virtually nowhere with a Republican supermajority strong enough to make it likely that they'd get the top two spaces usually), it gives Republicans, particularly (which are usually the vast majority of non-Democrats) a say in which Democrat gets elected, skewing overall results further to the right.
When you see a lack of accountability, it's mostly a choice being made by people who don't care enough to vote bad politicians out (look at primary turnout rates).
