I have been taught by RSA/OCR Word and Text processing. As a touch typist it is MANDATORY to always have DOUBLE SPACING after a full spot.
I have done a training in web design and still use double line spacing. Those without formal training to be a typist would not know about correct typing.
I am currently doing OCR Word and Text Processing Level 3 and Audio Typing at Level 3 - on COMPUTERS. I passed RSA Level 1 in 1995, it has changed to OCR.
As the OP said, it is far easier to read double spacing than one, especially if you want to skim vast text. Much easier to see where the sentences end!
I will always keep to double spacing. If more people did my training, this would not even be an issue.