It may only be part of a "marketing strategy" as you say, but it still illustrates the fact that having things open for use leads to further creativity and production of art.
Putting the vocoder itself aside, there's far more to the scene than the voice itself. Had this only been a piece of voice software without a character attached, it wouldn't have been nearly as successful. Evidence of this is scene in the prior releases of the Vocaloid software, such as Lola (http://vocaloid.wikia.com/wiki/LOLA). While this reinforces that releasing the designs into the wild was a marketing move, it also illustrates that the openness of the designs lead to a burst of creativity and production. Even for those that never produced any music using the software, it provided opportunities to participate in the community and contribute in other ways, such as illustrations, animations, instrumentation, etc.