XML import/export would be nice as well, but that also requires user interaction via the GUI (unless there's an API method like xml.import, which would be amazing). Admittedly, this would go a long way, but would not resolve all cases where API access is needed.
API access to the value mappings is not just about configuration. In fact, XML import is likely to be a better way to accomplish configuration. However, If I query Event history for an Item that uses a value mapping, there's no good way using the API to query that mapping for proper interpretation. The extra bit of labelling that gives context to the raw data is hidden.
I would say that the request for API access to the value mappings is much more about reading what's there and proper interpretation of the Event data than anything else. Of course, once there's a "valuemap.*" set of methods, adding .create, .update, and .delete methods is probably not too much of a stretch. But the .get method is probably the most crucial.
In my humble opinion...