I have been reading the stories in this thread for a long time, and I have realized how fortunate I am never to have run into weird groups like these - at least, until last week, when I spent some time leveling my Sith Juggernaut tank.
Flashpoint: KDY
Starring: Marauder
Supporting Cast: Mercenary Healer, Assassin Tank, Juggernaut Tank (Me)
We proceed through the two scenarios without any deaths, although the Marauder comes within a sliver of his life when he tries to duel a strong akk dog. The final boss is Lt. Krupp, who throws the sticky grenades. Before we can check to make sure everyone knows the mechanics, the Marauder jumps in and starts the fight. I type in a hasty "left console first" and join the fray. Soon enough, the first grenade throw comes, and the Mercenary and I run for the degaussing station. However, we notice that the Assassin and the Marauder continue to blithely smack away at the boss, taking the explosion when it occurs. I quickly type out an explanation that they should join us to cleanse themselves of grenades, thinking that they have merely forgotten this mechanic in the midst of their glorious DPS rotations. However, neither the Assassin nor the Marauder cleanse themselves for the entirety of the fight. The Assassin dies once, but miraculously, the Marauder survives. After we kill the boss, I type out a more comprehensive explanation of the fight mechanics in case both of them are completely new to KDY. The conversation went exactly like this:
Me: [grenade mechanics explanation]
Marauder: Lol muaraders have enough dr its uneeded xD
Me: that is untrue
Marauder: Only does about 10-15% my hp lawl
Me: if you say so
The FP had ended, I hadn't died, and I didn't feel like arguing with him. I just find it ironic that the heavy armor-wearing Mercenary and Juggernaut (Immortal spec, no less) felt the need to cleanse themselves, while the medium armor-wearing Marauder was perfectly happy eating the grenades. Then again, he didn't die at all, so he may have been on to something...