Admittedly the cog in this example is only subtly different — it's a graphical element that hasn't been affected too badly by the rescale.
But look at the edges of all those price buttons for example. On the scaled down version, they look really blurry because the edges are now no longer on whole pixel values, but fractions of pixels.
One might not perceive this to be important, but in using apps where the details like this haven't been sweated, things just don't feel right — as Jonathan points out, apps feel cheap and rushed. Even non designers will notice this, though they might not be able to specifically point out what's not right.