Paladin
Runic: Protective Lustration
cast time: 1.75 seconds
780 mana cost
390hp group HoT (4 ticks)
8% spell damage reduc
Shaman
Runic: Spiritual Attuning
cast time: 6 seconds
850 mana cost
600hp GHoT (4 ticks)
50% snare
20% slow
So yes, the high mana cost vs size of the hot might make this spell situational, not something you would use all the time in every single situation. But still 8% less dmg from all spells can be pretty damn good, especially since this can be cast on tankgroup and all characters that are tanking, ie not only useful for mobs that ae.
As a comparison the shaman spell also has a great hindrance. Sure it costs more mana, but heals for more per manapoint spent, but still, the 50% snare and 20% slow component pretty much prevents the shaman from using this in tankgroup (ouch on agro) or in melee group (ouch on dps). Spamming it in either of these groups would make you not so popular. This reduces its usability to caster group or life and death type of situations. Casting this spell in an exp group might not be very popular either, it would definitely not be something you would want to spam if you have a tank that depends on melee for agro, or a melee class at all in group. Even with these limitations from what I've seen of it it appears to be an awesome spell.
If using paladin spell in a groupscenario, you might not be using it during trashfights, but if I was on the paladin I would definitely load it up for most namedfights if I had a weak healer or the encounter was not trivial. In a group with a druid + shaman for healing or only one of those, a 140base heal difference (a 56% increase from ethereal) on entire group each tick is huge and for one short fight, the mana shouldnt be an issue.
Sure, the paladin spell is situational and not for every situation, but it's still a really really good spell for many encounters. If it wont stack with the shaman spell, that isnt really a problem, because the paladin will likely be in tank group while the shaman will be knocked over to the caster group, meaning these spells wont collide anyways. When this isn't the case, its likely that the shaman is busy slowing and single target healing a tank.
1.75 seconds isnt a lot of time for casting a spell that heals group for 1560 base dmg + gives an 8% decrease to all incoming spelldmg, this is a lot for a class that isnt considered a healingclass, in fact its almost as much base as shaman relic heal, likely a lot more when taking the 8% into account. I haven't used this spell myself yet, but from the stats of it I dont understand why you arent thrilled over it.