Why NRF24L01+ modules set RPD bit when detecting a signal above -64 dBm on the channel?
The restriction to -64dBm and also the 40us (I think?) delay is mainly so that people don't try and build an AM (de)modulator with the NRF. It's, in theory, able to decode packets at down to -95dBm. And yes, you can use it to find out which channels have ongoing traffic by tuning in on the channel, waiting long enough and reading the RPD bit.