Since I can not use JRiver's DSP on MQA, why in the whole world should I then take the detour via JRiver to stream from Tidal?
If you do not have a MQA capable DAC that you want to do the decoding, then you can use JRiver's DSPs.
If you want to pass the MQA signal to a MQA DAC for decoding you are correct that you cannot use the DSPs. However, by using WDM you do not have to worry about which software has exclusive access to the device. If MC has exclusive access, Tidal will not be able to play directly to the device and vice versa. WDM just simplifies that part of playback. If that is not an issue for you, WDM gains you nothing.