Sethmartin hit the nail on the head. The Jedi Order and the Sith Order are actually force religions. So its not actually light vs dark its my belief system vs your belief system. LS SW are viable because their quest for power whether they achieve it via the light side or dark side is still the same. DS Jedi however, I feel arent so realistic because they would most likely be expelled from the Jedi Order. Being a dark side Jedi doesnt make you Sith either. Because you would have to learn the Sith teachings first.
To who brought up Revan and Galen Marek. Galen Marek was never considered a Sith because there was already two Sith Lords, Sidious and Vader. Teaching Marek the inner works of the Sith Order would have broke Darth Banes "Rule of Two". Revan declared himself Dark Lord of the Sith after finding the Star Forge after being ordered to find it by Lord Vitiate. He in essence created his own faction and Empire separate from the faction we play as in game. Thats why on the Dromund Kaas questline the Revanites are branded heratics.
So yeah I totally went off on a tangent but I love SW lore.