My basic strategy is to ignore sentry guns, defenders, everything that gets in my way in the pursuit of the enemy flag. Sometimes a whole team, without exception, sits solidly in defense. Sure, Spies go on defense. You head for that exit spiral as fast as your little cyberlegs can carry you, and head downward until you engage the enemy flag-carrier. Scouts are able to chase Scouts, and that's why they need you against incoming Scout flag-carriers.
Scouts are also very very useful in other more subtle ways.
For instance, they are great Spy-detectors at the flag, because any Spy's disguise is instantly stripped away, and enemy Spies revealed, just by passing close to the Scout. Therefore, a Scout ahead of the flag defense, particularly the sentries, which are so vulnerable to the Spy's grenade attack, would be a wise precaution for the well-defended base. It is well to note that the Spy's Tranquilizer Gun can have a deleterious effect on a Scout's speed.