@MartinSleziak well at least they served some good purpose. :-) It's a bit odd the undeletion abilities; in particular we really cannot undelete self-deleted comments of other users. But we still can see them.

A reason why it can be handy to be able to undelete is that we also have the ability to delete all comments. In some case it can be more efficient to delete everything and then UNdelete selectively.