It's true that in a lot of the movies and books we don't have much in the way of a light side sith. But we do have a two interesting examples:
1) Darth Vader; in the end he kills the emperor, saves his son and returns to the light side. However, imagine that Darth Vader doesn't die at the end and instead negotiates peace with he Rebels as the new rightful Emperor. Some would see his killing of the Emperor as his ascent to power, while others would view it as his redemption. He wouldn't be a Jedi again, but he also wouldn't be the maniac Palpatine was. He'd be in essence a light side Sith.
2) Mara Jade; once a member of the Emperor's Hand to the a wife to Luke. She clearly starts at the depths of the dark side and evil and moves to a completely different alignment. At some points in her life, could she have been described as a light side Sith, I think so.
Next we have multiple accounts of Jedi falling in the heat of battle, only to go back to their more centered perspective. I think this shows that neither side is absolute or that you lose power using one side or the other, it seems instead they are just better fitted for different occasions, light for peace and dark for battle.
1) Obi-Wan clearly gets angry in his final clash with Darth Maul.
2) Luke clearly goes berzerker on Vader only to pull himself back together.
Finally I submit to you the groups that neither accept Sith nor Jedi teachings but use the force, showing that it isn't a necessary dedication to the ideals of these two groups that grants the power:
"These [groups] could belong to either the light or dark side, or simply exist somewhere in between. Some of the more well known examples of independent Force-user groups included the Ysanna, the Witches of Dathomir, the Aing-Tii Monks, the Baran Do, the Fallanassi, the Jensaarai, the Krath, the Sorcerers of Tund and other Force-based Organizations. Also, some Force-users did not adhere to any code or creed, but were self-taught or raised and trained by independent masters. There was also a family of Force-wielders known as "The Ones" on the planet Mortis." - Wookiepedia "Force-Sensitive"
So in conclusion, it is true that often the polarized Light vs Dark is the focus of a story within the Star Wars universe there are many shades of grey and different circumstances. I don't think a light side Sith is any less "realistic" than a dark side Jedi.