Dragging and dropping images into the inkscape document always creates an absolute link to the image file.
The import command has a different behavior: it creates relative links for images in the same directory or a child, and absolute links for the images placed in different directories. This is, IMO, the right way to manage image links.
Both image importing procedures should have the same linking strategy. Relative links for images are very important when working through LANs (for example: I created a brochure in my hard disk, and my partner has to do some changes on it).
Having relative links and images placed in the same folder than the svg allows to edit the file from any location without having to re-link the missing images.
There's also another related problem. When the image is imported from a samba share in linux, the link is always recorded as absolute, no matter if it was imported from the import command or it was dragged and dropped.
If the image is in a remote location, it's only imported with a relative path if the remote location is mounted with cifs (via fstab).
This issue wouldn't be such a big deal if the image properties dialog had a browse button instead of a text input for the path (Bug #172162)