The Scout is optimized for the anti-air role. Compared to the wraith, it has greater durability but no cloaking device, and is slower without the Gravitic Thrusters upgrade. Both have strong anti-air attack to counter heavily armored flyers such as battlecruisers, and weak ground attacks. Ground anti-air units counter the Scout.
The Valkyrie's diffused attack is less effective against the Scout.
The Scout is rarely used in competitive play due to its uneconomically high cost, the aforementioned weak ground attack, and the paywall the Gravitic Thrusters upgrade represents. Instead, niche roles are filled by the corsair.
The Scout, or at least its model, was originally a speeder bike.
In the StarCraft alpha, Scouts were armed with disruptors. In the StarCraft beta, Scout engines produced green exhaust instead of their current blue. Scouts also used their anti-matter missiles against ground targets.