I have played thus far exclusively with randoms and have yet to run across a single instance where i was griefed. I think some people are greatly over exaggerating this issue. Assholes are always gonna asshole, and there is literally no incentive to griefing. They're not just hurting you, they're hurting their own xp and money by lowering the overall reward at the end of the mission. Why invest 20 to 30 mins into a game only to shoot yourself in the foot on the reward at the very end? If people don't want you on their team you've probably proven to not be ready to play at the level they play at, are dumb, an asshole, or they just want a slot open specifically for a friend. If you're having a bad time with certain people, just block them and move on to one of the other 5 million people currently playing the game. Bitching about your bad experiences (incorrectly, I observe) is not a good look.