Here is something I worked on recently. Let's say you could re-parse code while executing it. A lock is needed so you don't modify a statement list while executing it (you don't want execution to see the partial list). This is where you want one mutable reference to the statement list and then change it completely...so you could use an immutable list at this point. But you could just as well create a new mutable list at this point and save some cycles.