That is one of biggest fails of TOR (and most games). In TOR, for lightside Sith you are actually a traitor. And darkside you are... a traitor. The difference is that in light you are doing it willingly, and in dark because you are stupid.
Example 1. On Korriban you may save Jedi. Going light means that you inform him of Sith manipulation and betray the Empire. Outside of some RP why would anyone playing Sith want to betray the Empire? Evil is not informing him. What the hell is evil this? And what good is in informing the Jedi? Your actions are based on politics (betray the Empire or not) and not on moral choices.
Example 2. On Balmorra you may choose between killing a Republic general or not. Of course killing him is evil, not is light but... NOT killing him serves the Empire more then going dark and cuting that guy down. If you go light you give Empire a valuable prisoner. So by the ex.1 it should be dark - you are advancing the interests of Empire. You it is light because... dunno.
So by those examples, if you were to play good Sith (good - loyal servant of Empire) you should be grey: lightside choice for Balmora (not killing general) and darkside for Korriban (not informing Jedi).
Yet there are no rewards for going the middle road.
Most play dark or light. I play light healer sorc (Sith Red Cross), but the more I play the more I'm inclined to simply go gray and choose whatever choices will be logical for a loyal Sith, good servant of Empire (and his own interests). Cause currently I play a traitor to the Empire and I DO NOT LIKE IT (you hear me, Bioware?).
Dark and light is NOT about serving the empire or republic, its about morals, dark choices are always morally "wrong" and light choices are correct. Killing someone is dark, regardless of whom he is, lying is dark, etc...
You dont seem to grasp what the system is based on at all :P