Well, it's just I haven't met anyone that got either NWN or HL and didn't play any mod for those. And in both cases, mods provided more entertainment hours than the vanilla game. If that's not depending on mods, I don't know what it is...
I'd say that given the insane amount of (good) modules for NWN, it's probably one of the games that gives the most bang for your buck overall... not to mention MP possibilities.