You have an excellent point; the world would be a much better place if we defended each other from insults, slights, and discrimination.
There are times, however, when coming to another's defense will not do any good, and may very well do more damage than simply leaving it alone. One person making insulting comments is an annoyance. A dozen people loudly shouting back and forth about someone else is far, far worse.
I'm not saying you should ignore other users making insulting or degrading comments. But make sure your defense is welcomed before you make it. Give the offended party a chance to handle it their own way. It's better to quietly offer support, and make a vocal defense only when you are sure it will be appreciated, than it is to speak out of turn and possibly make a situation you know nothing about even worse.