The problem is, the turnouts for primaries are pathetic and the establishment will often run one or more "ringer" candidates to further split the non-establishment vote to help ensure that their annointed one wins.
This would be cured overnight if everybody screaming for pure conservatives were to come out in force during the primary election and back the right guy. But all to often, they are sitting home at the very time their vote matters most.
Sorry. I was just pointing out that voting for the lesser evil also comes with its own down side. It is usually less unpalatable than voting for the greater evil. The Party did run ringers to split the non-Corker vote.