Sword of Spirit
Legend
Honestly, some people without mental health issues concentrate slightly better when they are multitasking as well.
Another good point I had forgotten about. If I recall my psychology courses, one scale of human psychology (something on an extraversion to introversion type of scale) tells us that some people's focus is improved by a greater amount of stimuli, while other people's is improved by less stimuli.
This is why some people are better at studying when listening to music, and others can't concentrate with the music playing. Another good reason to allow some people to multitask.
The big problem I haven't figured out a solution for is how they can do it without distracting those on the other end of the scale. Because it can be really annoying to other players and DMs alike to see some people apparently tuning out. Regardless of psychology, if someone in your group is wearing ear buds and nodding their head to the music coming through their mp3 player, it's probably not going to enhance your game experience.