I agree that that specific use case is a pain, but I don’t think federated logins are the fix for it. Rather, links to posts on other instances should be automatically translated to a link to the federated version of that post on your home instance, such that you can interact with that post without having to re-log-in. There’s a bunch of issues in the Lemmy GitHub project related to this, so hopefully it gets implemented soon.

In my opinion, federating logins kind of defeats one of the main purposes of federation though, which is to give the user control over where their user information lives.