1) It gives a lot of pleasure to respond with it to idiotic ideas.
2) It adds emphasis and a sense of proportions to the error, ie. it's not just a simple mistake, it is an idiotic mistake.
3) It (hopefully) shames the perpetrator of the idiotic idea into thinking twice before speaking.
It's not always appropriate, but a lot of times it is just the thing.
1) It gives a lot of pleasure to respond with it to idiotic ideas.
2) It adds emphasis and a sense of proportions to the error, ie. it's not just a simple mistake, it is an idiotic mistake.
3) It (hopefully) shames the perpetrator of the idiotic idea into thinking twice before speaking.
It's not always appropriate, but a lot of times it is just the thing.