The game very much works in cycles.
Not more then six months ago, anything light side was being crushed by Zual and his crew. Now, it is light side running everything. By the looks of it, NS will be really changing things up possibly,
Equal farming of light and dark side characters is important yes, building a deeper roster is important, but when you started the game late, it is harder to catch up on it all, that is with out even looking at the mass release of characters in the last few months as well. With the new movie less then two months out, I am going to expect more releases come the new year.
I use to be stressed and tried power farming against people that have been playing for a full year more then myself. In the end, just do what you can do. I brought up TB, because if you are in a guild, it is something you want to be working towards, to help the guild out over yourself. If you are not, I would not worry about things then, and just farm what you want to farm. We know dark side TB is coming at some point, so what you farm now in the dark side, probably will be helpful to you then.