Again, players should always have a choice in the matter. Think about allowing sharing to enemies in a 2-team game. One might say "the game is clearly intended for one team to defeat the other and not let one player ruin it by sharing to the enemy." But one might see a 3way FFA the exact same way. Ruining the game by sharing to an enemy and doubling his army, effectively ending the game can seem just as bad as in the 2team game.
If you're playing in a FFA game you need to expect to be attacked by multiple players. If it's one player controlling two coms or two players, it makes no difference at all to the player being attacked. FFA games are very random by nature, this is something you need to get used to, not something you need to prevent. If you want predictable games, go play 6vs6 BA DSD.
I personally prefer not allowing sharing, especially now that CA has an interface for ceasefires and teaming up against stronger players during the game
It makes about no sense that you're against sharing yet you support teamwork, sharing units is an important part of teamwork.
Ok, like CarRepairer suggested, i've made the no-sharing option now voteable, so the players can decide before the game what they want - but default stays "no sharing to enemy"
Pretty much useless if you allow ingame allying btw, you can just ally, share then unally.