[skn0tt]

Then what do you do about the old version of the mutated object? There will still be references to it in other parts of your object model, which will now be out of date. I think that the concept of objects preserving identity is central to OOP, which simply contradicts immutability.

[pas]

Could you give a concrete example?

Why would you somehow pass objects around that are not "final"?

I mean the objects that get pinned here and there are specifically designed to serve as wrappers for some state, hence they encapsulate that state, and thus they are mutable. (For example a DB connection/manager object, a service registry, a cookie/localStorage repository.)

Where the immutability usually helps is with shuffling data around, parallel computing (concurrent access, no need to lock), etc.