As Serial Ganker points out, most people playing World of Warcraft aren't doing so until something better comes along -- they're doing so because they enjoy playing the game. The players who are lumped in as "tourists" are people who, more often than not, had already left the game and were looking for a new game to call home. This, in no small part, is why it's hard for games that clone WoW to find an audience, as the people who are looking for a new game want something different.
The ultimate conclusion is that the idea of a tourist shifts the blame from developers to players -- that the game was fine, but people just jumped ship because they really wanted to go back to WoW anyway. It's a convenient myth, but really, we'll all be better off if we just put the right head with the right skeleton.