If one's primary concern is party politics this might be true. However, if one's main concern is creating a system that ensures the will of the people is the primary concern and not the will of the corrupt establishment primaries need to be open, just like general elections.
My question is why does this only apply to Democrats? They are more likely to include independents than not when decision left to the party and especially more than Republicans and most places the rules are the same for both parties.
So why does that only negatively reflect on Democrats?