Comment 4 for bug 1570074

@JazzyNico - I'd consider it a bug, or maybe more accurately an oversight at the time full support for 'preserveAspectRatio' was introduced in the new renderer, and bitmap import (directly, or e.g. via PDF) was adjusted accordingly (for direct bitmap image import in r13020, for bitmap images of imported PDF files in r13023).

See also:
http://thread.gmane.org/gmane.comp.graphics.inkscape.devel/42630/focus=42662

Note also that current trunk / 0.92pre includes an extension which can be used to add & set the attribute to bitmap images in files created with older Inkscape versions (Extensions > Images > Set Image Attributes) without having to mess with SVG code in the XML Editor directly.