If that is what GP meant, then they used the wrong term. The term usually used is "Blanket Primary" or "Jungle Primary" (Jungle Primary is a specific type of blanket primary) for what you describe. An open primary allows people to vote regardless of party affiliation; the states which have these may not even ask you for party affiliation when you register to vote (Virginia, where I first voted does not, for example).

It does make an aggregate difference because few people vote in primaries anyways, and even fewer will change their party affiliation every year, making raiding much harder.