Jarmo: Roleplay-wise both ranger and rogue tend to be similar light armor sneaky sort of types, so it's a good match.
I fail to see how the technicalities of which class combination you used to get your chosen abilities have any bearing on the RP of the character. It's the abilities you have, not which classes gave them to you, that defines the character.
I suppose if you want to pursue the animal empathy line some of the campaigns (SoU chapter 1 in particular) do support that, but that's about the only place where there will be any significant distinction between what a Fighter/Rogue and Ranger/Rogue can do. Otherwise, you're just giving up combat feats for very little in return.
Jarmo: Light armor and dual wielding are something very natural for a rogue, and favored enemy "humans" is always a good bet.
The benefit is very small to begin with. An epic ranger who has a big bonus against a bajillion enemy types is one thing, but a non-epic ranger is getting meager benefits. For a non-epic multi-class ranger, you're unlikely to have more than a +2, meaning weapon specialization is just plain better anyways.
Jarmo: Beside the actual skill points, I seem to recall ranger has more "roguish" set as class skills than a fighter, so there's that as well.
Since NWN1 allows you to carry over as many skill points as you like between levels, this is essentially a non-issue for anyone multi-classing Rogue.
Jarmo: I've had great fun with Barbarian/Rogues (Conan!) and Fighter/Rogues (city scoundrel) characters as well.
Depends on the kind of character you're making.
Barbarian/Rogue works great. Paladin/Rogue is also a fine combination. They both do unique things that a Fighter/Rogue can't. That's my concern with Ranger/Rogue; there is precious little he can do that a Fighter/Rogue cannot do just as well or better.