I think it's a completely unnecessary thing to say and it sort of treats the person you are talking to (or at least someone within earshot) with pretty large amounts of disrespect. If you really care that some one does not hear what you are about to say....don't say it.
Further, if you are the person that doesn't want to know...walk away. If you really don't want to know, make it so that you can't know! You can probably detect that something is coming up that you might not want to hear, so act on it.
It's a lazy way of saying either," I acknowledge your opinion but I don't care about it so I'm telling you so that you can't blame me", or "I'm not strong willed enough to keep this information from myself, and will choose to blame you for not respecting my desires." I've even noticed people saying it to themselves.
The other thing that we are saying a lot is "Not to scale." It is the scientific/misconception equivalent and again, it annoys me.
If something is not to scale...Why? and if not, make it to scale. It really isn't that hard to do. It also sort of depends on the scale that you are talking. The picture of planets and our Sun for example. It is to scale (if you are looking at sizes), but nearly every time I see this, someone says, "this is of course not to scale."
|Exactly to scale|
Worse. You don't need to tell me that it's not to scale. I can sort of see that! Especially when we are talking about very large things like stars or universes, or very small things like molecules or atoms. I think the assumption can be made that what you are dealing with is things that are not to the scale of what they actually are. Or at least not a 1:1 scale.
When talking about a film or TV show or something and I know that someone is listening who might not want to hear. I won't say anything. If I did in error, and the person gets annoyed, I can always come back with, "hey, does it matter, it's a TV show!"or something.
When it comes to scales, we generally do explain or communicate things that are to scale, otherwise it would be useless to explain them that way.