I think you underestimate the average consumer. I understand that anecdotes are not data, but let me share one here anyway - this may be specific to the India market
I visited home (India, Hyderabad to be specific) recently and my sister happened to break her GS3 Mini. I didn't want to buy her an expensive replacement because I plan to get us both Nexus 5s in January, so we went shopping for a cheap phone. Obviously, the Lumia 520 is better for her since all she does is call, text and Whatsapp, and she can keep it for a spare phone once she gets her Nexus. Like many people have said here, the salesman actively dissuaded us from buying a Lumia. Why? The return and complaint rates are extremely high. One major reason for this, he said, is the lack of a file manager.
Thing is, in a market like India your smartphone is more often than not your only computing device. People don't always have desktops or laptops, even or should I say especially students. They need this ability since the phone is their USB flash drive, media player, storage, everything.