It should be allowed to create a AP Group without any WLAN. In this case, I'm working on a configuration for R-Series routers that sets them up as dedicated ATAs. I am also interested in a configuration service pass-through devices that provide the customer with an ATA, and allow them to plug their own router in to the R-Series device. These sorts of configurations need the WLAN to be disabled.
Right now, when attempting to create such a thing, an error is displayed as seen here: