Eliminating range folding error in doppler radar returns
Frequency

Most high-power radio-frequency amplifiers (klystrons, cavity magnetrons) have a very limited frequency range.  A dual-frequency radar would essentially be two radars hooked up to the same antenna.

Polarization

Horizontally-polarized radar waves reflect more strongly from the ground than vertically-polarized waves, while circularly-polarized waves reflect only weakly from rain.  Consequently, vertical polarization gives the best results for weather radar.  A dual-polarization radar would sacrifice detail to eliminate range error.