It's just that we can make the cloneArray fast, by using Array.prototype.slice
and yet back compatible, by using the current approach when the second argument "callback" is provided.

Also, when any other improvement comes int the future (e.g. from) we'll just change implementation 
of Arr.cloneArray()...

So, I don't understand why we are actually making in obsolete/deprecated.
(note that one more function call is not a big deal at this point for me, which could be an argument)

It's perhaps that I don't understand the rules used to decide what API should or should not be
marked as obsolete.

Honza