I use this to top people off when I cure them to barley injured. When you've got the healing domain, 6hp isn't useless. If I empty my orison slots out with this, I can give back 36ish hp. Not terrible when you're trying to conserve heals and keep everyone in top form. Is that OP for a 0 level in PnP? Yes. In NWN? No, its just the others are under-powered. But is it as powerful as being able to copy a book with one 0 level spell? Or make light in the absolute blackness of dungeons (which has killed me more than once) Or detect if something's poisoned? I'm not sure.
As Irdni said, you get max HP at level as well as less spell slots. But also NWN differs from table top in one HUGE way. There is vastly more combat. In PnP a single battle with a group of monsters might take 3 hours. In a night of gaming, you might see three or four encounters. You get hit far less often, go through far less HP. If we brought healing in line with PnP, we'd also have to nerf the healing domain to be pointless and useless, were spells are treated as 1 level higher (thus giving you 1 extra hp per spell). No one would ever... ever... take the healing domain again, and healing would not be a viable strategy in combat. We'd be straight back to only one possible option for combat... ward to the moon you super AC tanks, fight.. rest when the wards drop.
NWN is not like PnP, doesn't play like PnP we have to remember some changes are for the better. If anything, address virtue by having it give up to 4hp.
Very small potatoes. Healers at very low levels are nearly useless, and without cure minor... holy moly they would be like lips on a chicken.