I left Sith pretty early to work on MilitaryRP when it first came out, so I didn't get to fully experience the "degredation" phase. While I was Sith with Joah, Jackson, Kitty, and Tyzen I had the most fun I've ever had on the server.
Due to the fact that I, like many other players, have been here since the beginning, I have reached the top rank or near it in every full time-faction, with at least 4 months of active playtime within each category (Naval, Jedi, Clone, BH, Senate, Sith). Because of this, I have very little interest in working my way up in those factions yet again. Just based on this alone, it seems like it would be great to re-add sith for people like me. Even though it would be a re-addition it's something "new" to do, and would likely pan out very differently than it's original addition to the server.
However, I have strong doubts that the Sith Faction could be successful in the current state of the server, so I'm going to have to -1. Everyone else pretty much explained my thoughts. In my opinion, adding Sith now would just de-stablize the Roleplay dynamic that High Command works to control.
I think it would be great for a small group of dark-side force users that could operate in a similar function to the CIS faction. Dooku could even oversee them so we don't have to add another applicable leadership role. I don't know if this would work out either, but I think it's a better alternative to the full addition of Sith.