Artesia, moving targets are that much harder to hit, so unless you're standing still while shooting at the spy, chances are you'll probably win.
In general crits are to promote attacking, if you're doing good and you're not close to dead, why not go on a rampage? It makes a siege more effective because you get that chance to roll that lucky crit that will deal more damage.
While crit stickies and rockets can kill just about any class in a single shot, they are not one-shot weapons and can be somewhat easy to counter if you're pyro, seeing how you can airblast those projectiles away. Headshots are not nearly as stupidly easy as Counter-Strike, so that's point one, point two, you have been endowed with the ability to strafe, meaning you can step aside. A sniper gets a clear shot at your head when you run in a straight line, so be random, especially if you know he's there. Similar thing with spies, except it's easier, since they're usually running backwards. Only part of the situation is in the other player's control, you cotrol another part, the rest is up to luck and skill.
In general crits are to promote attacking, if you're doing good and you're not close to dead, why not go on a rampage? It makes a siege more effective because you get that chance to roll that lucky crit that will deal more damage.
While crit stickies and rockets can kill just about any class in a single shot, they are not one-shot weapons and can be somewhat easy to counter if you're pyro, seeing how you can airblast those projectiles away. Headshots are not nearly as stupidly easy as Counter-Strike, so that's point one, point two, you have been endowed with the ability to strafe, meaning you can step aside. A sniper gets a clear shot at your head when you run in a straight line, so be random, especially if you know he's there. Similar thing with spies, except it's easier, since they're usually running backwards. Only part of the situation is in the other player's control, you cotrol another part, the rest is up to luck and skill.