I admit I have fed them and even shown others how easy it is to get them to land on your hand with food. I don't see a huge deal with it personally, but the article you read probably discourages it because of the type of foods we feed them. Some animals don't digest "human" food well or aren't built to get the nutrients out of things other than their natural diet. So while you may think you are helping them, you might not be doing them any good in the long run. Plus, if they get too dependent on humans, they might not do as well on their own when the humans aren't there.
I've never seen them be aggressive to the point where I think they would bite me or cause harm, but they certainly do get brave. It takes a few landings on your hand to get used to the grip/texture of their feet. At the top of the state where we snowmobile, there are a few places and intersections where you know as soon as you stop the jays will be out and looking for handouts.