Each faction that does zone control, does it with in the same time frame each day they do it.
So a spy sitting on your exit grid sending a report back saying, "Yeah, they are lining up on the exit grid now. About the same time they did yesterday, and the day before, and the day before" is hardly news.
Each faction that does zone control, does it with the same crew each time they do it. So when the spy reports that 10 or 12 people are lining up, that isn't news either.
The number of power armors reported can be useful, but really how they are deployed when the zone control is actually happening is more important. This is especially true since power armor doesn't rapidly come into the game. If a faction had two power armors yesterday, they are likely to be rolling with two today. Essentially, most of the information that a spy at the exit grid can give, is already known.
For a spy to be truly effective, they have to be a part of the zone control group. A spy in your crew will do ten times the damage that one at your exit grid can do, since they will not only have a clear picture of what you are currently doing but also what you are planning. If they are part of your zone control, they can tell the other side exactly where everyone is deployed in the zone, where all the scouts are located and even give advanced notice which zone is targeted BEFORE your team gets to it.
Is a spy sitting on your exit grid cheesy? It sure is, but it isn't all that dangerous to you. A spy in your zone control party, on the other hand, will ensure you fail and fail miserably.