This is from a strictly defensive purpose. Scouts obviously have value for scouting and scout bombs. That being said for farming and building cities (cities where you keep food upkeep to a min), I've come to the conclusion that scouts are worthless.
To be relatively un-scoutable, a city needs at least 500k scouts and a good hero. However, if an alliance wants to get a report its still possible with a coordinated effort. Also the scout bomb / scout technique is effective against smart players who close gates during scout bombs. Finally, almost every alliance has a spy which means a potential attacker can send 1 scout attack and have the spy send the report. So trying to prevent someone from getting a report is pretty much a futile effort.
Now, consider the cost of scouts. Each scout costs 5 food while swords are 7 and archers 9.
In my experience the average city has about 200k scouts which is an upkeep of 1 mil per hour. For that price you could have ~143k sword or ~112k archer. Your city is less likely to be attacked with the extra swords or archers then by trying to keep someone from getting a report.
