I hear the phrase "junk miles" too often IMO. Depending on usage I don't believe there is any such thing as "junk" miles. If "junk miles" are considered to be low intensity efforts or "just riding along" then IMO they have a very valuable place in the riding/training world, at least as far as I'm concerned.
I use them as 'recovery' or 'active recovery' miles/rides between hard effort days. From what I've read, rides at this low level of intensity are important as they promote strong blood flow through the muscles, provide gentle stretching and movement and in my case also focus on 'suppless' or smooth, effortless pedaling cadence.

I guess if that's all I did and then complained about not being able to go fast, long, etc. they could be considered as 'junk miles' because they are being used in place of targeted training but as I use them they are very important to me and my training program.