A launcher with a command that does not have a %<FOO> in it still accepts a DnD from Thunar, but then silently fails. In the properties there's no notice about that behavior either.

I'd recommend to either not accept the DnD if there is no %<FOO> in the command, or to at least print an error to stderr (even when DEBUG is not set!).

Also it'd be nice to have a little note about this somewhere in the launcher properties. I could imagine a little button with a question mark, which would pop up a dialog that contains the description of the %-options, similarly to what Thunar shows when you configure the custom actions.