Puddy - pretty much all decoders manufactured (and AFAIK since the early DCC days) are dual mode. In all of the ones I have worked with (Bachmann, Digitrax, TCS, Lenz and possibly some others) the quality of C control is inferior to what is available in DCC. I am assuming that you are asking specifically for motor control functionality as that is pretty much what is controllable in DC mode. The are workable in DC but the control is not as precise as in DCC.
What functionality to add or remove? None. There really aren't any options to be added or removed from what is available in DCC standard (while running on DC). You only control the motor speed. There are no convenient means to access any other functions in DC. Some manufacturers advise to turn the dual mode off because on some DCC systems the model in a dual mode can take off on uncontrollable full speed is DC mode is enabled. I think that manufacturers is Digitrax.
You also didn't specify weather you are looking for sound-less or sound decoders. Sound decoders in DC show even higher degree of inferiority (compared to DCC control). Most have very poor throttle response as is range of speed control is usually only from around 6-9v to 12V. So you are cramming the full range of models speed (from stop to full speed) into the upper third (ore even quarter) of the DC throttle know. In DC most sound decoders play some automatically generated sounds which are in DCC controlled by function keay in the throttle, and some will play appropriate braking sounds. Some decoders will play a whistle or maybe a bell if the DC polarity is flipped quickly enough to prevent decoder shutdown, but using sound decoders in DC is really an exercise in frustration.
What to add? My recommendation would be if you run DC and have a sound decoder, spend $140 and get yourself a basic DCC system. Problem solved.
The only real use I find in the DC mode of DCC decoders is when I perform a basic test whether the model is alive. I touch a 9V battery to the wheels to see if it will wake the decoder up and run the motor (that utilizes the DC mode of the decoder).
