Well here's why I don't do that.
Let's say I'm using white fog, on a black background.
If I have objects that are fogged by 50%, and I don't alter the transparency of the fog as I alter the transparency of the object, then what I'll get is objects fading to a white outline as I fade their alpha out.
Now, if my background was white, which it would be if we were talking about real fog, this would look right.
Or would it?
Sure, it seems like it would work correctly on first glance, but consider what would happen if you had other objects behind that object you were fading out.
They would have 50% fog applied to them as well already, but now your object is fading out, but the fog affecting its color is not faded out with it, so now you have a 50% transparent white siloughette of your object affecting objects behind it even though it is completely invisible.
And that's obviously wrong.
So to make fog look right with semi-transparent objects, you have to alter the amount of fog on the object as it becomes more transparent, such that the colors will remain at 50% saturation throughout the whole fadeout process. And that is what I've done here.
Let's say I'm using white fog, on a black background.
If I have objects that are fogged by 50%, and I don't alter the transparency of the fog as I alter the transparency of the object, then what I'll get is objects fading to a white outline as I fade their alpha out.
Now, if my background was white, which it would be if we were talking about real fog, this would look right.
Or would it?
Sure, it seems like it would work correctly on first glance, but consider what would happen if you had other objects behind that object you were fading out.
They would have 50% fog applied to them as well already, but now your object is fading out, but the fog affecting its color is not faded out with it, so now you have a 50% transparent white siloughette of your object affecting objects behind it even though it is completely invisible.
And that's obviously wrong.
So to make fog look right with semi-transparent objects, you have to alter the amount of fog on the object as it becomes more transparent, such that the colors will remain at 50% saturation throughout the whole fadeout process. And that is what I've done here.