I use CoW a lot just managing files. It's only an efficiency gain if you have enough space to do the data-copying operation. And that's not necessarily true in all cases.

Being able to quickly take a "backup" copy of some multi-gb directory tree before performing some potentially destructive operation on it is such a nice safety net to have.

It's also a handy way to backup file metadata, like mtime, without having to design a file format for mapping saved mtimes back to their host files.