As a matter of fact, explaining why someone is wrong is generally less of a waste of time than insulting them.
Consider the benefits: they might learn things they didn't know, you will have a better understanding of your own feelings, and you may both get some insight into a different point of view than your own. These are, in my opinion, more valuable things than the fleeting satisfaction one gets from telling someone off.
Suppose it takes five times as long to write a decent comment as it would an insult. Would you rather look back at some point in the future and say "I wrote 50 insults", or "I wrote 10 good comments"?