No reason the spell can't work differently for different specs, other spells function like that.
And I really don't think Disc survivability is that amazing, I can live significantly longer on my Druid and I'm not even that good at Druid, just because between Shadowmeld, Displacer Beast, Travel Form and HoTs I can't really be touched. And from what I've seen/heard Paladins have about the same survivability as Disc between Bubble and two BoPs plus sprint/freedom. The trick to kill Disc Priests is to spam Dispel, if you take off their shields instead of damaging through them they have no chance. For example Frost DK/Prot War completely dominates my Priest because of their dispels and stuns, impossible to heal through it for long even with decent peels.
Of course, if I have a Ret on my team I'm virtually unkillable, but that's not the same for every healer with a Ret on their team heh