Obviously different raids will place emphasis on different things.
In my experience in this guild, people are placed in progression fights strictly based on who will give us the best chance of killing the boss. As a boss becomes more and more farm status, the focus shifts to who will make the best use of the items that can drop from the boss, for the sake of future progression kills.
There are no 'favorites'; performance and attendance override social ties and seniority. For example, we sat our GM on our Blackfuse progression when another of our healers logged on simply because our GM had seen the fight less due to a break he took.
Just going to mention that since roots break on damage almost instantly, I don't really consider it worth the GCD to use on mines. I run vortex on that fight instead, since it both slows and has a knockback-like effect.