Whereas signal attenuation through radiating (leaky) coax is comparable to standard coax of the same dimensions and general shield construction, the most significant factor with radiating coax is coupling loss.
If one considers a traditional UHF band antenna, the coupling loss - the transfer of energy from one medium (current in a physical conductor) to another (electromagnetic wave); in this case, the mathematical ERP value in dBm from the antenna to a point in space generally measured at one meter - will be about 20dB. In other words, whatever the effective radiated power level calculated at the antenna, one meter away will be about 20dB less.
Coupling loss with typical radiating coax tuned for UHF band will be about 78 to 82dB at one meter for 90% coverage (meaning that the radio system as fed by the radiating coax will provide 90% reliability as compared to an antenna at the same field strength). Coupling loss is such a significant factor that it is a standard specification provided by manufacturers of radiating coax. Given the tremendous coupling losses, radiating coax is generally used only in long narrow applications: hallways, transportation tunnels, train platforms, mine tunnels, catwalks and the like, where the portable radio device is never more than a defined distance from the coax. With modern engineering and manufacturing processes, radiating coax can be tuned rather tightly for a given frequency band(s) as well as a defined radiation pattern, both of which can reduce the coupling loss by 3dB to 5dB.
Using radiating coax in non tunnel/hallway type locations usually does not work well unless a significant amount of TX power is provided, or very careful filtering and pre-amplification for RX, is utilized, and even then it will rarely be better than a proper antenna(s).
But in long hallway or pedestrian tunnel, radiating coax works great, given proper initial system RF gain structure. And yes, Jason, diplexing RAD outbounds and inbounds over the radiating coax works very well.