I found this to be an interesting read that is appropriately non-committal in my opinion.http://www.shrm.org/Publications/hrmagazine/EditorialContent/Pages/4Zeidner-Music%20in%20the%20workplace.aspx
It is definitely not black and white, would most likely depend on the person, where they sit, type of work, what they are listening and so many other compounding variables that to say "studies show lack of focus when listening to headphones" and then say no one can wear headphones is ridiculous, "studies also show improved focus when listening to headphones"
different studies, different workers, etc. My point is this can really be debated either way, again not a black and white, yes or no, many variables at play. Taking that decision and autonomy of allowing workers to choose how they work and what makes them more effective is surely not good for morale.
The study does seem to be pretty definitive on headphone use as a means of employee retention, maybe that some research worth sharing with management...???
Another thing that often comes up is headphones being used for signaling to others they do not wish to be disturbed, which I find of great benefit in many offices.