Yet it is also perhaps worth pointing out that there may be a fourth school of thought regarding languages which is even less well-acknowledged - it's not so much implicit as subconscious. And it's this: if, like Tolkien, you conceive of fantasy as being a kind of "legendarium", then it may be entirely appropriate to approach the construction of fantasy languages and words as being an exercise in communicating meaning rather than in being realistic. A good example of this is the use of words in Mythago Wood; I don't believe for a second that the words used in that book are at all accurate as examples of old Brythonic or whatever, but the word urscumug has a great ring to it as a word for a kind of hellish guardian monster of the forest. Similarly, I would say that the word orc has a certain feel to it for English speakers which is suggestive of what the word "orc" represents. Fantasy languages, in other words, have a role to play in creating a mood for their audience that is representative of what that audience understands "fantasy" to be.
It is suitable and appropriate, then, to think of fantasy words as having to communicate mood to the readership rather than making sense from a linguistic point of view. Yet the observation that there is a poverty of imagination there also holds true: if you want to be interesting and different, you could do a lot worse than listen to how people in other parts of the world (especially its more obscure corners) actually communicate with each other.