It would be of limited use, but it would be great if the Modbus driver could “map” a device on the first connection. Basically I envision sending a single read request to each register and checking for exception code as the response. This would be really useful for getting data out of “black box” devices in the field without the trial and error of adding addresses until you hit a read error.
What would this give you? You still don’t know what the data means. It would just be a bunch of numbers 
This is one of those features that would be nice to have, but would only be used rarely, and mostly by integrators.
For example if I’m integrating an existing device into a system that has no manuals, or the manuals are really vague, I could hook a signal generator up and watch the outputs. Having the modbus driver enumerate the valid registers would save time reverse engineering the outputs.
Also, you probably would be able to get the device to enumerate in the OPC browser with generalized labels…