Bill Tindall, E.Tn.
What reason would anyone have for doing a single side mortise? If you can make one, why the reticence to do a double?

One, they are trivial to do. And if they are not regarded trivial then some training and a small amount of practice will make them so.

Two. Given that they are trivial to make, the ease of accurately locating the lid or door to the case when using a double mortise would make it by far the preferred technique unless there was a compelling reason not to.