A great way to deal with scaling assets for mobile is to create your art in a vector format to begin with. For Uncle Slam on iOS we did all the artwork in Illustrator and exported to PNG at three sizes: iPhone 1x, iPhone 2x (and iPad), and iPad 2x. The results are sharp images with the right level of detail and no scaling artifacts.
We even took the exact same artwork and made an 8 foot tall banner with it, and it looks great!
You assume here that size doesn't matter, when it might. If the difference is only initial download size in your game, and is a few dozen MBs, then yes I agree. but for some applications, this is content that is loaded in runtime, and since size DOES matter in this regard, some applications will need to upscale images.
We even took the exact same artwork and made an 8 foot tall banner with it, and it looks great!