It matters. You have to hold lots of double resolution images in memory, for example, which obviously eats memory up fast.

I wasn't sure how Retina vs. non-Retina apps behave in memory. Some apps presumably ship both sizes of assets, in which case the OS could load only the smaller ones into memory. But if an app only ships Retina assets, would those go straight into memory, or is the OS clever enough to resize them dynamically and throw that into memory?