There are many limitations to the palantiri, that would make them poor for military uses. The main problem is you have to know where to look, and if Sauron was planning to launch a surprise attack, how would the person viewing the palantir know where to look...it's supposed to be a surprise.
That's just one of it's limitations though. These aren't clear, crisp, perfect images. The palantiri rely on light and shading. When Pippin looks into it, Sauron thinks the palantir was still in Orthanc...that is why he asks for Saruman and says he will be sending a messenger. Sauron had no idea Pippin was camping outside somewhere in the middle of Rohan, well removed from Orthanc.
Another problem is it takes a significant amount of will to even look into them. Tolkien in the UT writes that the Stewards eventually became rightful owners of the palantiri (as were Elendil and his heirs). As the rightful owners this gave them an advantage over someone like Sauron, who purely relied on his inherent power to use them. However, the only Steward whoever dared to use the palantir was Denethor. And he had spent most of his life as Ecthelion's heir studying all the records and lore about them. Denethor had begun to use the palantir after his wife died, and there are two different strains that Tolkien talks about which Denethor under goes. The mental strain of simply using the palantir, which caused Denethor to look physically aged, and the strain Sauron causes when he discovers Denethor using it. The combination of the two strains would eventually end up breaking Denethor's mind. Aragorn who also was a rightful owner of the stone, doubted his strength when he uses it to confront Sauron, and that was barely enough. Aragorn was pretty drained after using it.
So imagine someone without the mental abilities of someone like Sauron or Saruman, or someone who was not a rightful owner, as Aragorn or Denethor were trying to use it. Plus, not to mention Tolkien points out in the UT that Aragorn and Denethor, had a strong mental will, which was necessary to use the palantir even for being rightful owners.
The only benefit is that the palantiri cannot be made to lie, however they can be manipulated, as Sauron was able to do with Saruman and Denethor. Denethor didn't discover the Corsair ships, or Sauron's numbers, Sauron was essentially feeding Denethor images that Sauron wanted him to see. This increased the despair and hopelessness in Denethor, which led to Denethor losing his mind.
All in all, the palantir would be poor objects to use for military purposes. The person has to know where to look, they can unintentionally lead to bad intel simply because the images aren't clear and crisp. Afterall it led Sauron to make the mistake of believing Saruman was well and good in Orthanc and just with-holding a treat for Sauron. This error, caused essentially by the palantir's unclarity, leads Gandalf to take full advantage of Sauron having to play catch up and discover the error. Also, the viewer has to have a strong mental will to even use it, whether the viewer is a rightful owner or not, simply because of the strain it puts on the person looking into it.
It's interesting, despite the nature of the palantiri to not be able to lie, they are actually best used in acts of deception. Sauron does it with Denethor, and then Sauron himself is deceived by the palantir because of Pippin and Aragorn.