Thanks for the thought-provoking post!
I do find it very important for us — data viz people being in this field for some time — to reflect if we just get carried away by our passion or is there an obvious value coming from us. If the latter is true we should be able to getting the point across very clearly. I will address my opinion about one of the many goals of data visualization, namely exploratory analysis.
My take on this is the following: Analytics is about finding relationships in data. To make the analytical results useful and applicable in a certain domain, we need to combine the patterns which we might uncover in the data with the domain knowledge we already have. Since in almost all domains the existing data grows exponentially but still does not cover parts which we humans know, the analytical goal is to leverage knowledge in our heads together with the patterns we find in our data.
Combining data, its discovered patterns and our knowledge is the basis for successful analytics and facilitating the communication between the user and the data/computer through effective interaction mechanisms is exactly what exploratory visualization tries to achieve.