The argument is that if you wear gloves you miss out on the early signs of alarm from the bees, and therefore they are more likely to swarm and get aggressive towards bystandards. Without gloves you get a better sense of the alarm state and can avoid problems. It's generally easier to manage alarm state without gloves.
I got my first hive a few years ago, and I am noticing that there are fewer recommendations of working without gloves in urban environments today. I don't know why this is the case. Also I am used to a large yard/small town environment (lots of pedestrians, folks doing yard work, etc), and there are plenty of other environments where I would expect this to be less of an issue (for example a rooftop hive on the third storey is going to provide a lot less exposure in this regard). So demographics and hive placement may have something to do with it.