I don't think Sith Warrior PvP is completely useless. In fact, depending on your AC you can do quite a bit.
Sith Juggernaut - I'd suggest mainly speccing to be flag carrier/flag support in huttball. In Alderaan and Voidstar, I find that protecting healers and managing crowds works well.
For Marauder - I'd probably suggest taking advantage of cloaking and picking and choosing your targets. For example, in huttball you notice that a healer is constantly healing the flag carrier. You can cloak and then selectively attack this healer to annoy him/kill him.