Depends entirely on the subject.
Some threads get annoying if there is a decent discussion that goes wandering off. Some threads are fantastic when it happens. Can you moderate it in a place like GD? No, because you would kill it. A few threads being derailed for the worse is a small price to pay for some of the comedy gold you get around these parts.
In the technical/hardware subforums a derailed thread can be really, really irritating. I find it generally doesn't happen much though.
If you want to change the moderating, ban such threads as "I shot myself in the leg, what do I do?" or "Spec me a woolly hat" or "I have some steak that smells like a thousand rotting sheep's heads. Is it safe to eat?"
+1 excellent summary.