One word, Instagram. It's become a social fad. It does have roots in Polaroid photography which tried to capture casual moments instantly, on low-fi instantly developing film. This look carried forward in to the filters of Instagram due to it being a legit cultural reference to Polaroids.

From there, the vintage feel of it took on a life of it's own for it's ability to easily hide imperfections and to make things look classical. It then further got magnified by becoming the latest version of social status updates and became a cultural fad.

As mentioned, there is some potential artistic justification for it in the realm of instagram since it is intended for the Polaroid type style, which focuses on low-fi photos with an emphasis on capturing what is happening, as it happens, in life. This is the goal of instagram.

Where things went horribly, horribly wrong is when that fad expanded into the traditional photography space and people started applying it to mediocre photos that do not follow the old Polaroid shooter mantra. Instead, what you get is a crappy filter on a crappy photo that doesn't fit the artistic context of the shot. There are times when a vintage look does help an image speak its message, but the vast majority of current uses do not match the design language of the vintage look, but rather slap it on to capture some of the social popularity.

The problem with doing such sell-out techniques to capture some fleeting popularity now is that it will cause the image to age as soon as the fad changes. Right now, the casual viewer may see a low contrast image and think it looks cool simply because it matches up with the fad, but a few years from now, as the fad passes and it becomes clear to everyone else that it has been overdone and overused, those images will become recognized as the irrelevant, trashy gimmicks that "snobby" real photographers will already tell you they are.

The images that used it properly will survive as classical examples of the era and the ones that used it for a quick popularity boost will be forgotten. If you want to make truly great, timeless, memorable photographs, you shouldn't use gimmicks to play to the cultural fad of the day unless they fit. In other words, use a low contrast style of post production not because it is popular, but because it adds to how your image tells the story. If it doesn't add to the aged and rugged feel of your content, then don't use it.