Yes I agree that polygamists would be regarded as heretics by many who formed and signed the pledge.
They believe in traditional marriage, the trouble is that their tradition doesn't go back far enough
But apart from that, I see it as a useful step in the self-awakening of parts of the church, in regards to the depth with which it has entangled itself with the state.
also as I read more of that article, I see that they are more about making a statement of their religious separation, but still implicitly expect their newly-church married couples to waltz on down to the courthouse and sign the government papers.
they don't discuss the possibility of a church-married couple not proceeding with the implied second step.
I wish they raised it as a hypothetical. I mean, what if a couple got a church marriage, and wanted to go to the courthouse but there was heavy snow and it was impossible to cross the town. So they stayed the night in the motel next to the church, then went to the courthouse the next day. did they sin that evening? [see how wicked I am to have thoughts like that]
seriously though, the question for the pledgers is, are their church couples actually married (yes of course we know they are, but maybe they don't); and if the answer is yes, then it raises the question of the necessity of the second step.
but as for me and my house, we will never get a state marriage again.