I've found the idea that people use their headphones with one ear in and one ear free absolutely preposterous and by some extension a mind glut or non commitment to a task at hand.   

If you must have one in, is the content consumed worthy of your attention? If not take it out, if so put both in even if it is background noise. If someone needs your attention have them come to you or be ready by not having them in.  

If it’s a matter of situational awareness why have one in and compromise 50% of your awareness in an environment that needs 100%.  Why not give an area that needs your attention what it needs. If not kick back (mentally) and enjoy the music.

There is a level of splitting attention that comes with this that people were simply not ready to take on. Either be one with your thoughts or throw on some tunes and drill down into a task or situation. 

I understand that this is not a popular opinion as many people will as stated above use one headphone to listen to music whilst doing a mundane task but still keeping an ear open. I simply say to you put both in and bust it out or be attentive for who you need to be attentive for.

Don’t half-ass your listening experience.  You paid good money for your headphones. Enjoy them.