I agree that primaries are more important for conservatives to get involved in in order to win. For example, Peter King could be taken out in a primary by a small but dedicated group of people and a halfway decent candidate. There are about 150,000 registered Republicans in his district. Republican primary participation is around 10-12%. With the closed primary system, you only 9000 votes against him to knock him out. This could be done under the radar with a few thousand dollars and a lot of door knocking and telephoning.
Peter King would be totally destroyed by a conservative third-party candidate in the general election if he’s not felled in the primaty.
And in his case it would be worth it.