Why disguise as scout?

I've seen quite a few videos on Youtube where spies repeatedly switch to a scout disguise after each kill. Slow scouts seem like obvious spies. Why not choose a Pyro, Engineer, Medic, or Sniper?

Sometimes using an obvious disguise has its advantages. One example is if you are using the dead ringer. Being obvious can trick people into attacking you and activating your cloak. The enemy might think he just killed a crappy spy and then POW backstab!

One reason is that scouts are often seen running around lots of different places on the map, unlike say the engineer, who can normally be found near his buildings. So if you're a spy disguised as a scout, your mere presence in a certain area isn't as likely to arouse suspicion as if you were a different class.