Even in EvE, if you happen to microwarp drive into someone at like 3000 m/s, you'll just bump their shields and end up going right angles in some direction or the other. If you're driving 4 gattling guns at a target, and your cutter mate jumps in front of you, you'll waste ammo. That's pretty much it. I dont think wasting ammo gives you any advantage over someone in OPEN wasting their mate because they stepped in front of you... If you somehow continue shooting your mate, and accidentally fire a few salvos of torps for good measure, that's pretty much on the person with the trigger
All this will still happen in OPEN if you wish to practice mopping floors with a friends corvette.
In Mobius OPEN, you're there to co-operate. Lore can suggest that all commander weapons have to abide by a convention that they're tuned in to the same shield frequency or hull modulation that renders them inert at time of impact. An ED Geneva convention, if you like... How ever you want to role play it, it can be made viable.
Friendly fire ON only gives griefers the ability to smash the game mode ethos on a silver platter. It will do more harm than good ultimately. PvE is by nature PvE, not PvE whilst someone empties salvo after salvo of plasma into your back, no mater if they're friendly