In FA, scouting is very important for a variety of reasons. You need to see how advanced your opponents economy is, where their commander is, what type of units they are making, etc. But the biggest reason you, or at least I, scout is to get advanced warning about snipes, experimentals, and nukes. This can be very tricky, especially on large maps, where it can take a spy plane 30-40 seconds just to get over the enemy base. Once it arrives there you have a small window of about 4 seconds to view what units he has and the progress of things like a nuke launcher or artillery piece. This is irritating because often times you cant watch your spy plane go over the enemy base as you are busy managing your units and base. This often results in me missing mercies, fighter-bombers, and experimental units because my plane dies before I can see them. I think this might be a big part of the reason why even pros are vulnerable to snipes, they cant reliably get a clear view of what units the enemy has.
This is a little off topic, but I have problems with the idea of fog of war in the first place. You could justify it by saying that its "realistic", but from a gameplay perspective I'm not sure it makes much sense. Every good strategy game should force the player to make decisions based on what their enemy is doing. Since, logically, decisions need information to be reasonable you should always be scouting no matter what. If the game practically makes you scout if you want to win, that means it is not really a choice and thus not "strategic." In other words, it wastes the player's time by making them spend effort on something that doesn't require thinking.