The Dark Nuclei have a tendancy to aggro onto a healer if left unattacked, however this is not guarenteed. There is also no confirmable reason why they aggro onto the healer based on all observations of how they function and most of the time they will just float in place and not aggro on anyone.
Having a healer tank Keleseth effectively means three things...
1. DPS can't do anything to Keleseth because of threat generation of a healer. A healer generats 50% of effective healing as threat, distributed to all hostile targets. Even if you're doing 12,000 eHPS, that tops out at 2,000 threat per second based on healing to each of the three bosses. That isn't a feasible amount of threat generation for an ICC level encounter, DPS would simply be unable to attack one of the easiest targets for them to attack.
2. Healers do not have high effective health, as their priorities are towards high mana pool instead. Even in PvP gear, it would be unlikely that a healer could have more health than a DPS, and no where near the health of an actual tank.
3. The healer that is "tanking" can't heal at 100% efficiency. Because they would be forced to move to aquire Dark Nuclei. No healer can sustain maximum throughput while moving. Additionally they have to use non-healing spells to insure that the Dark Nuclei does in fact aggro onto them. Because their effective health is lower than a DPS or tank, they will also require more Dark Nuclei in order to survive the empowered phase.
Because of these three reasons, it is never feasible to have a healer tank Keleseth. Most raids will use a conventional tank on him because of the highest effective health. However, if effective health is high enough, a DPS can also tank him. A healer can not generate sufficient threat, have the same amount of effective health without greatly hindering their healing, and heal effectively on the move.
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.