That was bad advice as others have said. It is also against the law but luckily it is not enforced much.
I do admit that I will go against traffic if it is safer to do so. I know of one 11 lane stretch of road that I have to go past where it is just plain scary trying to cross to the other side to go in the correct direction. Instead, I stay on the side I am on and ride against the traffic until I can cross at a safer place.
To ride against traffic at all times creates a number of dangerous situations. When people are in a parked car and are about to exit, they check their side mirror and open the door when it is clear. They are not looking in front of them for bicycles going against the flow of traffic.
What do you do when you see another bicycle coming in the correct direction? Who is supposed to to go around?
What about vehicles backing out of driveways? Drivers check for traffic coming from the correct direction first, then glance at the sidewalk the other way to make sure they are not running over a pedestrian. They are not expecting or looking for a bicycle coming up at 15 mph going the wrong way.
Also, if there is road debris, lawn clippings, etc. in the road, then when you go around them the closing speed for the cars is going to be much greater. So drivers will have even less time to slow down or go around you. A vehicle behind you traveling at 40 mph will have a closing speed of 25 mph if you are doing 15 mph. That closing speed is 55 mph if you are going the opposite direction. That means drivers have less than half the time to react to anything that you do going against the traffic as when you are traveling with traffic. I don't know about you, but I would rather they had more time to notice me rather than less.