That I agree with. That’s way too late to end a primary. You would have to have confidence that you were going to win, and essentially build a good campaign staff FOR the primary that could follow you into the general election.
On the other hand, if they had their primary back in the spring, Castle would have won easily, as we were still in the “we need to win elections” (see Scott Brown), and not into the “we are going to win big, so we should try to get real conservatives” mode.
A September primary also doesn’t give the candidates enough time to heal the wounds of the primary campaign, or for the losers to get over it and back the winners.