An explanation is given that in case the UFO was a spying spacecraft of some enemy, they don't want that enemy to know that his effort to spy them was detected. This seems to make sense. In 2011, Iran's nuclear facility was spied by US using a so called 'lockheed Martin RQ-170 Sentiel UAV.' US thought that Iranian radars could not detect it, but they were wrong. The Iranis detected it, but kept it secret. Finally they captured it via a cyber attack on US's system. So we see that Iran's choice to keep  observation of a 'UFO' secret helped them to finally capture the technology. So it is a kind of 'secretly spying on the secret spy'!

If we can capture even an ET spacecraft without their permission, then the better! But for this to happen effectively, we must fool the ET into thinking that we are not aware of his presence in our skies. This allows him to carelessly move around and to do so, more often! This is especially apt if the ET is a malevolent one!

But then since all technologies can be used for military purposes, this attitude will also stiffle our knowledge! If we keep something secret, then we are also denying the info to some 'unauthorized genius' who is not a member of the military clique! This attitude infact leads to a fear of everything that we don't understand that we might observe it. It leads to a denialism. So to me, even if there are some benefits in keeping the UFO knowledge secret,  the bad consequence of that outweigh its benefits!