Being mindful of your emotions is good. But suppose you have anger, and you steadily focus on it. If you mind wanders, you correct and return to focus. Well, you are now meditating on anger, anger has become the object of your meditation. Because anger feeds on itself, it is highly likely that this approach will strengthen and sustain your anger.
So what is this mindfulness of emotions that is good? It is first, being aware of the presence of anger, secondly, seeing the consequences of anger, and finally making a decision to either apply an antidote, let it be, or move on to the next thing. Of course it doesn't just apply to anger, it's just one example.
Things get subtly different if you focus on the nature of an emotion rather than the emotion itself. For instance greed and despair are impermanent, empty, and they are states of the mind rather than states of the self. They overpower rational thought and compassion and can even cause them to cease completely. Matthieu Ricard seems to think that such a meditation will deprive the emotion of its fuel, causing a fairly rapid extinction of the flames, and my personal experience supports this.
Sergeant Schultz knew everything there was to know.