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?
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.
As well as the other two points (Dead Ringer to get shot and Scouts are generally all over the map), I have my Spy-disguise menu set to short-hand notation, meaning instead of getting a list of classes with the numbers 0-9 to choose them, I get two groups of 1-3, the first chooses either the Attacking, Defending and Special class group, the second allows me to choose a disguise from that group.
So, when disguising quickly, I've found it easier to just hit 4-1-1, which is
- 4 - Bring up disguise screen
- 1 - for 'Attack' Group
- 1 - choose Scout
Which is where my fingers tend to sit naturally on the number keys. Sometimes if I'm thinking about it in advance, I'll choose 4-3-3 (Spy). When I'm actually trying to blend in successfully I'll choose more carefully.
The only reason why I choose the Scout disguise is that your speed increases/decreases with different disguises (ex. Heavy makes you run slower and Scout makes you run a bit faster than normal). I disguise as the Scout on long maps (such as pl_badwater) so I can get to the enemy faster. Plus, as said before by Colin D, you can trick the enemy into thinking you're a very crappy Spy. With the Dead Ringer (An Invis Watch that can fake your death), you can fake your death, sneak behind the enemy that "killed" you, uncloak, and backstab.