I think this is great concept and overcomes the issue of defining your position with a series of numbers and letters without any inherent check that the coordinate has been correctly received. It also provides an ease three word phrase to remember. However Send.Moon.Fish is very different to Sends.Moon.Fish. The former is in Essex and the latter in Fife which should be obvious of a mis-communication to a first responder. The other issue is that there is no hierarchy which means that Send.Moon.Fish is nowhere near Send.Moon.Dogs which is in Arizona.
From a Defence community perspective, if What3Words is being used by our potential partners then we will need to understand how it works and be able to covert such positions into a more traditional system that can we used in our systems and processes.