It hasn't always been so. I bet if the party closed the open primaries in the states they exist things would be very different.
I would tend to agree, but experience has proved the opposite can still be true. NY has had closed primaries for ages. I was a member of the NY Conservative Party. NY is infamous for its RINOs. I changed registration to vote for Thompson who dropped out before its primary. I had to change registration at least 25 days before the last general election in November 2007.