Pro666 is right. If a product isn't publicized, how is it supposed to sell. Even if you consider word of mouth as a good way to sell games, which I think it is, I still think that the game should be cleverly marketed in key areas of social (online, and offline) entertainment i.e. movie theatres, youtube, etc.
It also depends on the product itself.
Now, if Nintendo announced a brand new Zelda game even with minimal marketing push it would still sell because the brand name and the quality of the product would make it a must for everyone.
NMH, on the other hand, is a niche, crazy concept that could alienate some gamers, so even if Ubi marketed the HELL out of the game it would probably be a cult hit.
In these cases, word of mouth is the strongest form of marketing.
NMH might not have sold a million copies, but it sold enough for it to get a sequel (even with a piss poor Japanese performance).