Defederation should be a last resort when an instance is always making problems outside of itself or have illegal content. Otherwise it is better for the network to remain connected and not partitioned and for blocks and moderation to deal with problems. Respectful community members should be allowed to participate respectfully all over the network regardless of their views as long as they behave.

For example I personally am very critical of Israel, and would not want a Palestinian instance defederated, but at the same time, an Israeli instance deserves to be allowed to enjoy it as well even if the two are very likely to have different views.

Does it accomplish much? The same users can just create accounts on lemmy.ml or here.

The brigading is an issue, but defederating doesn't seem like a huge help right now.