That it came from Donald Rumsfeld in the context of what we know now and what he surely knew then is why it's such a good quote. The words basically say nothing but are also true about everything. So it can implicit be a warning that there is probably some bullshit going on or someone has a sense of humor and is also warning people while also avoiding the subject - of course just my opinion. How people actually use it will depend what the audience agrees it to mean.