That's too general and too dogmatic. "Always" doesn't include the 1940s and 1950s when liberals were actually fighting against Communists, and "whenever" doesn't have much to do with the days when National Review was edited by John Sullivan, when they published Sullivan's and Peter Brimelow's criticism of immigration policy. Fringe voices have been excluded from National Review or movement conservatism, but it's easy to forget that exclusion wasn't immediate or automatic. Plenty of dissidents found a home there for longish periods of time.
For example, why did a vociferous defender of Senator Joe McCarthy believe that the anti-Communism of the John Birch Society was beyond the pale? Also, why are the seemingly benign sociological observations of Jason Richwine worse than the anti-civil rights editorials from the early days of National Review?
There were some real snakes that Buckley excluded from his movement, whatever his own past was like. Not everybody who criticizes "movement conservatism" from the right is a good influence. Isn't that clear by now?
National Review movement conservatism is quickly becoming a thing of the past. What you'll see now is people trying to define their own version of "movement conservatism." They'll behave in the same high-handed way as the old NR commissars they criticize and exclude ideological deviants in the same way that they condemn yesterdays' movement poohbahs and panjandrums for doing.
You're right about the tolerance-if you could call it that-displayed by magazines like National Review. I think Brimelow was a senior editor at NR until 1998-quickly jettisoned once Lowry was put in charge.