That still leaves the other limitations, which I would stress more than first knowing where to look. The palantiri were mysterious and perilous objects, even in the hands of the rightful owners they were feared to be used. Why do you think Denethor was the only Steward who ever dared to use it? It is because Denethor believed he had the will and strength to do it. In some ways he was accurate, but mostly he foolishly over-estimated his strength, which leads to his madness. Aragorn feared to use it, and Gandalf continually stresses the danger.
They are objects that people should all in all, stay away from, and it should only be used by 1. the rightful owners and 2. those who were strong enough. Even then, they should only be used in the most dire situation, as when Aragorn decided he should reveal himself to Sauron. The bottomline is they were dangerous objects that someone needed to know how to use, but most importantly not abuse. With the palantiri it's like a "look at your own peril" kind of thing, because if you under-estimate the dangers who knows what you will wind up doing to yourself.
To me, that makes them poor for military purposes, much better, safer, and probably would get more reliable info just to use commando Rangers and spies.
