Yeah, now you just have to get that information to your own troops...You could still see enemy troops manuevering and the like.
Only occasionally, because you can only keep a limited amount of fuel up there.No one has mentioned the most practical use of them yet. Signaling. Over long distances, balloons would be excellent to convey information to other portions of a kingdom or the like.
Really, the hot air balloon only becomes useful when you can come up with a more useful method of communication than shouting. Also, how much damage could a ballista bolt do to a hot air balloon?