I loved it in custom games when people just got bored and started killing each other. Fun times. Oh and will there be custom games and forge?
Bungie hasn't commented on a Map Editor mode or Custom Games yet. I feel that both are possible, the former might fall outside the scope of the first game, but I'd be surprised if the latter didn't make it into the final game, though I don't expect to see it in the Beta.
Nope, friendly fire is off, because its about working as a team, not about being a d*ck because ya bored. No forge mode, which makes sense in a always online context, as for custom games, no official word, not impossible.