A spy need not be trusted by the enemy. Stealth, Bluff, Disguise Self(wizard 6), and the Hallucinatory Item ritual all provide ways for a spy to observe planning sessions without the targets knowing that an untrusted individual is present.If the enemy is a different race, you'll have trouble finding a spy to insert, and even if you succeed, it'll take weeks for them to work their way into the enemy's confidences, whereas casting a scrying spell takes an hour.
An informant can be acquired by kidnapping. Discern Lies permits an easy way to assess their reliability.And if you have no lines of communication to the enemy camp, you can't acquire an informant, and even if you could it'd be difficult to assess their reliability.
True enough. Consult Oracle is only good if you already have a fairly clear idea, and can ask precise questions, or if a rough idea is sufficient.Consult Oracle will gain you sketchy, possibly ambiguous information, and relies largely on you knowing which questions to ask.
Indeed it does. But who they are and what they are doing can be ascertained in less than a round.Scrying, uniquely, lets you see your target first-hand, giving you a clear, unambiguous view of who they are, what they're like, and what they're doing.
What they are like can take longer to figure out, depending on what they are doing during the scry. However, the people who develop psychological profiles for law enforcement agencies can tell you that you don't need to observe a person to get a feel for what they are like.
What otherwise unavailable information do you gain by watching them for an hour, once you know who they are?