Here's a graphic representation as I see it:
The upscaled image is better than the SD image but its attempts to replicate what is "really there" in the HD image are off in a number of areas. You might not realise the errors are there if your eye is untrained, but when you compare the true HD image next to the upscaled image you can see where it is wrong.
So what, you say? That doesn't mean much in this case because this is just a collection of random pixels I cooked up, but imagine this was an image of 2 million pixels. There would be distortion on the image, some things would look fake and certain parts of the image would look fuzzy when they are supposed to have increased clarity.
Just because you don't care about HD doesn't mean that others don't and for those people an upscaled image will never be able to cut it.