I think nobody will judge anyone on the basis of stupid/irrelevant comments. Think of questions asked here as a sort of informal discussion in conference halls. Somebody asks something, and then back-and-forth comes a discussion from other folks to get the question right. Most of what being discussed has obvious answers, or is just imprecise, but we see nothing wrong with that. A hypothetical example:
A. I can't believe factoring is so hard!
B. Is it? Can't you just divide your number by 2, 3, ... .
A. Sure, but that takes exponential time.
B. Err, it just takes a linear number of divisions? Ah that's exponential in the input length. I see.
I personally prefer to see "boneheaded" comments because usually they're "boneheaded" only after you give them a fair amount of thought, and so the first thing that comes to your mind may be the first thing that occurs to many others as well, and this would make your quick (but obvious/stupid) comment helpful after all.
Answers are a different story, though. They must be as precise as possible. If there is a bug in your answer you should fix it. If it can't be fixed, you should delete the answer to avoid confusing people.