Friendlies are fine as long as they do not jeopardize the flow of a match.
Their mere existence in a server can do this, as they take up a slot someone who shoots in an FPS could take. Now don't get me wrong, it's fine to take a break celebrating if you're like taken over Steel's point D (nearly-assured failure for RED) or standing on a CP with 6 people on non-KotH map or even high-fiving that enemy spy on humiliation if he was such a good chap, but sitting in spawn with the neat latest community taunt or dancing to the other side of the map or being out in the field throwing sandwiches to enemies is not what the game is about.
Addendum: Some players are even starting to exploit these people to win: I remember seeing a video where a guy disguises as a friendly Hoovy from the enemy team on 2Fort and eventually manages to capture the intel by crouch-walking with a Sandvich from the middle bridge to an unguarded intel room. All while disguised as a fake friendly player.
Poll