Defederating cuts off the whole instance. They just blocked those three piracy communities as far as I understand.

Remember that lemmy.world has to keep a copy of whatever content appears in a federated community on their servers, making them legally liable for the content. At least they just blocked the community instead of defederating.

A strength and a weakness. The strength, as you say, is being able to move to a different instance. However, the weakness is that Lemmy (the software) requires each instance to keep a copy of every federated post for its users to interact with. This means they have to host (and be legally liable for) data that they can’t police beyond blocking the community / instance.

There is no point to linking communities- if they are going to have identical content, just pick one or the other.

A better option would be for cross posts (using the Lemmy cross post feature) to exist as a single entity that is visible in multiple communities. This would allow for some differences in moderation which is the justifiable reason for multiple communities on the same topic in the first place.

Why do people insist that there needs to be (for example) /c/politics on every instance? Really, there are only 3 or 4 with any substantial traffic, and there are good reasons to pick one over the others, and they are the same good reasons for them to be separate.

There is a cross post feature, and the resuting post appears to be aware it was cross posted - it would be nice if Lemmy would consolidate those to one post that appears in multiple communities, or at least show you only one of them.