EmberJS uses strings as property paths. You can use 'foo.bar.firstObject.baz' as a unique key the same as any other function that takes a property paths (like a computed property). In Ember you write a lot of these so any small overhead like a lambda is a bit of a waste.

Also I don't think ember would work with functions instead of strings, the whole point of the strings is that the unique function is only called when one of the items in the property paths changes, and you can't get that from functions. You can only evaluate them.