I have heard that Clerics are really good in D&D 3.x; does this translate to NWN2? In particular, does a party with just Clerics work well?
Yes, yes, and no
The difference is that in pen and paper you can get around not having certain skills at your disposal. If you can't disarm traps, you can call an outsider who can. If you can't traverse your way through the wilderness, you might cast a spell to simply stride through the clouds and go over it. If you can't get through a door with a fiendishly devious lock, you can just cast a spell to open a passage through the solid stone wall next to it. These options don't work in NWN, and as such you're stuck playing by the game's rules, which is to say it's constantly going to be testing your skill checks. That is very punishing for the Cleric, since that's the only real weakness of the class.
You could definitely do just fine with an all-cleric-except-for-the-rogue party, though.
(Incidentally, in one game I've played, Wizardry 8, I have found that unbalanced parties (where you have one role over-represented) often work better than balanced parties, mainly due to the way that game is designed; melee and ranged don't mix well in that game because your party is all in one spot.)
In D&D 3rd edition, there are basically three roles in combat Damage
- defeat enemies by taking away their hit points Battlefield Control
- split up enemies and otherwise disrupt their tactics so your party can single them out and eliminate them efficiently Support
- buff allies and debuff enemies
Clerics can fulfill literally all the roles described above, and do it very well. This is why an all-Cleric party works so well. Now, it's not optimal
per say because you get a degree of redundancy, and there are other exceedingly powerful classes that are the equals of the Cleric. It should be noted that "tanking" and "healing" are generally not considered proper roles in pen and paper, just things your character might be good at, but Clerics are notably quite good at both those things.
Combat is only part of the game, and utility has to be considered as well. Their bad skill allotment is the only real weakness of the Cleric class, but they make up with it with a whole bunch of versatile spells. NWN doesn't do this aspect of the casters justice, as many of the non-combat spells were straight up removed. So this is a legitimate weakness of the class in NWN where in pen and paper it's actually a net strength.