I would guess that it's due to the fact that the image dialog code has a dependency on the xhtml text editor, so that have to be reworked before the dialog can be used outside of that context. I suspect that this constraint is gone with CMS6, but it doesn't seem like it has been enabled for the LinkCollection property in the RC though.
Not really much help, but at least some background...
Why is the clickable images property disabled? Are image links not supported?