Break ups are hard and as friends sometimes we run out of comforting things to say. It's easy to say, "he's a jerk and he never deserved you." or "she's self centered and rude, you're better off with out her." It can be difficult and tricky if/when they decide to get back together - yet it can be helpful in boosting morale if they don't.
Just look at Rachel Bilson and Adam Brody. One minute we think they're on and another we think they're off again - how frustrating! If you were Rachel's pal, and during this rough patch with Adam, would you trash him to make them feel better?