If they don't want us to use these classes, then they should take them out of the example code that comes with the SD library.
CardInfo is the first example in the list and it uses the SdFat utility functions exclusively ...
// set up variables using the SD utility library functions:
Does this mean that the SD classes cannot get the job done?
(Datalogger, Dumpfile, Files, and ReadWrite are using the SD classes)
Actually, it seems that using SdFat directly would be a better solution until the SD library improves.