Did you somehow mix up "Friendly Fire" and "No Friendly Fire" in the OP?
Anyway, I hope it is on by default, but can be changed if wanted. As already mentioned, "spy checking" is just way too easy without friendly fire. Teamkilling on accident shouldn't be a problem either, because on the one hand both factions styles are very distinctive and on the other, a panic-burst won't kill you right away.
? I don't think I mixed them up. Friendly fire = unable to harm teamates - no friendly fire = harm teammates. (What is OP btw)
And when I 'mentioned' spy checking, It is too easy WITH friendly fire (On), because you could shoot team mates without fear of hurting them, but any operative in disguise will NOT be protected by Friendly fire as they are not on your team. So if in doubt, WITH friendly fire, you could just shoot.
So IMO there shouldn't be friendly fire. (Can harm Teammates)
EDIT: Now that I think about it, saying No Friendly Fire kinda makes it sound like No harming Teammates
But I have always known Friendly fire ON= No harming Teammates.
I swear in games when you turn friendly fire on, you can't hurt your teammates... ahhhh My head hurts