Scouts don't *have* to be strung out when part of a non-scout formation!
Attached scouts can be used to limit the approach of the enemy to vulnerable units, can be used to force the enemy to go around a formation, or to limit the depth that support fire can reach into a formation.
Unless the enemy has infiltrators, units wishing to assault will have to eliminate the large scouts Zone of Control if they wish to get into close combat with weaker units. The large zone of control also means the formation can control more area on the battlefield, this can make for effective screens/pickets that the enemy will either have to destroy or go out of their way to avoid.
One way they can be used, though it's rarely done, is that if half or more of a formation is scouts, it can be garrisoned in the Tournament Scenario, *regardless* of what the other units are, this includes war engines! A local player would often bulk up a basic Ork Warband with Gunfortresses... and lots of Stormboyz and Kommandos, so the whole thing could be garrisoned for a rapid assault.
With more mobile formations, such as Eldar Aspect Warhosts, a few scouts thrown in can play havoc with their zones of control, forcing the enemy to move away from a formation they may not want to assault.
I'm sure more people will have more to say, but that's me off the top of my head.
