You can even try to drag from Nautilus (I wrote Krusader because that is what I use the most). So here is use case:
1. Open Inkscape.
2. Open Nautilus and drag some PNG file to the opened Inkscape window - it works.
3. Now try to drag the PNG file to the Unity launcher - Inkscape icon is disabled and you can't drag the file to Inkscape (so if the Inkscape windows is hidden you can't drag to it).
You can even try to drag from Nautilus (I wrote Krusader because that is what I use the most). So here is use case:
1. Open Inkscape.
2. Open Nautilus and drag some PNG file to the opened Inkscape window - it works.
3. Now try to drag the PNG file to the Unity launcher - Inkscape icon is disabled and you can't drag the file to Inkscape (so if the Inkscape windows is hidden you can't drag to it).