When I'm out riding, I take pictures of the various abandoned bikes I see.... I've shot hundreds of them. Some stay locked up for years; some get taken away by the city because they become eyesores, rob useful space at the bike racks, make snow cleanup difficult, etc.
Interestingly, probably 90% of them are mountain bikes of various kinds. Good brands, cheap brands, pretty much anything. What does this mean? I don't know... Either the usefulness of the mountain bike spills over into commuter duty in fine fashion, or they're the leftovers of the 90s MTB boom which ran out their last miles hauling Dad down to the train station until eventually they get left behind forever... I don't know which.