Ah, yeah, perhaps I should have elaborated more. It's a subtle issue, but it's one that comes up all the time when optimizing React by removing unnecessary ren

Ah, yeah, perhaps I should have elaborated more. It's a subtle issue, but it's one that comes up all the time when optimizing React by removing unnecessary renders. With immutable data structures, you know that x == y implies, even if x was initialized a while ago, that the data is the same. With mutable data, that's not true -- perhaps the values referenced by x have changed value in the meantime, so x == y does not imply that the data is the same.