While centrist candidates maybe preferred in Primary elections, but it is a choice that has been made by the people when presented with a full ideological spectrum by the range of candidates standing for elections.  Appeal to Swing voters is what matters in elections anyway so what Open Primaries do is make that abundantly clear, with the candidate most likely to carry swing voters in the general election most likely to win the candidacy. This makes party leaderships think hard about what voters want and how to incorporate that into policy.