i wouldn't have said that the ! is there because the designers want you to favour immutable rather than mutable operations. it's there for the same reason confirmations are there for deleting files; they want to you be sure you know what you're doing, because the immutable operation is always safe while the mutable operation is sometimes unsafe.

if the ! for mutable operations weren't there, people would often be very surprised by the behaviour of reverse, sort, etc because in languages like ruby (python, etc too) they expect things to be fairly safe by default.

there are plenty of cases when mutating an object is far preferable to doing the same in an immutable way for both speed, and algorithmic complexity.