Whenever I played Halo and went on a killing spree in multiplayer, nothing killed my buzz like a random teammate splattering with a ghost…
Edit: Personally as long as they balance it out, I'd like to see it in PvP
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In the one mode I care for, PvE I'm quite glad friendly fire is off, nothing would ruin the mood more then trolls who just go around killing others for giggles.