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.
The common use I'm referring to is similar to the OP, which is using it as a framework for assessing risk. In particular, aligning a team on the "known unknowns" is critical to building the confidence and alignment needed as a group to be able to deal with unquantifiable/inestimable risk.
I just took a look at that wiki article for Rumsfeld's usage of the "There are unknown unknowns" and I had no idea that he barrowed this phrase to frame his arguments and I was only familiar with that context, unfortunately.
