mattbilson/jQuery-Retina-Display-Plugin — comment on commit c4e45c1a4862e80380f1fe63b872e9811d944581
File:  line 0
By mattbilson:

Phonegap (and maybe other mobile app frameworks) must handle ajax HEAD requests strangely, as in my app if retina() was called numerous times (after adding content to the page via ajax for example), the image src would become example-2x-2x.jpg

Even without the Phonegap issue, if the retina() function is called numerous times, this reduces the number of ajax requests which are made to check if the image exists.
It does mean that a normal, non-retina image (which needs switching), can't be named with -2x. However this seems rare, and if needed, retina_part can be changed accordingly.