I think choosing between power splitters/combiners and multiplexers/demultiplexers depends on your system. A multiplxer adds signals and a demultiplxer divides the signals again. But in a power combiner, the signals are added so that in each branch of the splitter, we have an attenuated version of the whole signal. In a MUX there is no attenuation, unless you change the value of insertion loss from zero.