Reasoning behind leaving out off-hands:
- Off-hands have no armor. Armor used to negate only physical mob hits, which made it rather useless. In Cataclysm, armor reduces all physical damage. If an AOE ability or boss ability is physical damage you will take much less damage wearing a shield. I can think of one example off the top of my head which is Quake on Ascendant Council. Granted, you're not supposed to get hit by it.... but with a shield you can survive.
- Competition. With shields you compete with other holy paladins and with off-hands you compete with (potentially): paladins, priests, druids, shamans, warlocks, mages. The likelihood of the caster classes being pissed when you take the off-hand is relatively high.
- Adds. On the off-chance your tank isn't Chuck Norris, a few adds may hit healers while being picked up. Shield armor helps then. It's probably not enough damage that your self healing can't cover, but it might save other healers the trouble of getting your health bar up.
- Pride. Yeah, it isn't a good gameplay reason. So be it.
Are these reasons enough to not list off-hands on the BIS list? Are there reasons I'm missing? Let me know in comments.