Of course, not everyone agrees that an open primary is better than a closed primary. The political parties, in particular, want closed primaries and have sued various states to end the practice of open primaries.
Political parties want closed primaries because they want more control over who gets elected through these primaries. They argue that open primaries allow the other party’s voters to come and vote in their primary. For example, if there is no Democratic contest (often the case if the incumbent is a Democrat), Democrats can all vote in the Republican primary and try to pick the weakest candidate. Political parties feel that this is unfair.
People who prefer open primaries believe they are more democratic. Such primaries do not force people to declare themselves as members of a given party. They do not limit people’s choices as to how they will vote. Open primaries give the people the greatest possible freedom to vote for the various candidates that they support. This makes them more democratic.
Thus, we can see that there are arguments both for and against open primaries. The argument for the open primary rests on the idea that they are more democratic.