We should support the Chomsky et al letter for justice and open debate regardless of disagreeing with some of the people who signed it.
In other words, if you won't sign a letter whose points you agree with, simply because among the 150 other signatories are a few people that you disagree with passionately on some other specific political questions, you're practicing and fueling the harmful divisive approach of cancellation.
I excoriated J. K. Rowling's unjust lawsuit against readers in 2005,
and called for a boycott of Harry Potter in response. She has not apologized so I continue the boycott. Please do not use Harry Potter books or movies (or fan goods) in a way that will give her income.
Nonetheless I am glad that she signed this letter, because her signature will support freedom of speech. I would gladly have signed my name right after hers, despite my disagreement with her on the other issue.