Originally Posted by
Celentes
To beat a heroic encounter on progression, you need to have X raid dps and Y raid healing, also Z healing/raidcds during burst raid damage mechanics.
If the leader chose A number of healers and B number of DPS, you will eventually beat the encounter after you've learned how to execute phases and have gotten enough gear to a) to theoretically beat the encounter at all and b) to create margin for error since nothing ever is perfectly executed, especially on progression.
By not providing utility that your class has (in case the encounter demands), you're nerfing your raid team in terms of progression.