Bluetooth module (BLE112) USART: do I need to use its own microcontroller?
If you look at the block diagram on page 20 of your linked datasheet, you'll see that all of the I/O, including the USARTs, go to the 8051 controller. The 8051, in turn, is the only path to the Bluetooth radio section.

Therefore, I'd say that the internal microcontroller is required for the module to function.