Q: Why is bumping (writing comments under forgotten topic) bad?
A: If you write something under not updated for a long time topic it will jump at the start of topic list and will move down newer, more important topics. If the post wasn’t updated during months, there is possibility they just forgot about it and possibility that they don’t need to discuss it anymore (You can check this by previous posts). If you have something important to say — write, but make sure you will not disturb anyone with something like “I agree with you!” , there is 
button for something like that.