Outdated translations are marked like this.
Why is it disabled here?
Because of bug 11168! ;-)
There are several advantages:
- You can use your image on every wiki of the Wikimedia Foundation.
- It avoids having to re-upload missing files after importing all test wiki pages to the new site.
- Commons is specifically for files. Consequently it has a better policy, etc. Incubator doesn't have any control on uploaded images.