As someone who frequently roleplays an Imperial Agent around Sith, I understand that it is extremely important to play the role as a subservient and obedient character who serves the true rulers of the Sith Empire. On the other hand, I do find myself frequently being annoyed by many people who swagger around with the title "Darth" yet do nothing to command the respect of others. The majority of the Sith population either use my agent as a punching bag to show how evil they are with no other reason, use my agent to fetch drinks from the cantina while they continue to socialise and bicker amongst their fellow Darth-socialites and some even treat my agent as a friend and an equal, trying to get him to refer to them by name and to "loosen up a little."
After enduring this for a long time, my opinion is that people should only use the title "Darth" or even "Lord" if they're willing to represent the characters to a degree of accuracy. A Darth is one of the highest ranking individuals in the Empire, what are your responsibilities? Who forms your power base? What allies do you have and what enemies do you have? If you are a Dark Lord of the Sith, what do you have to show it besides the fact that you've reached Level 50?
I'll continue playing my agent as I have: Obediently following the Sith so long as these orders don't conflict with those of other Sith or involve putting his health at risk for nonsensical reasons. My point still stands however, many of these Sith that my character has to follow are woefully inadequate and would have never left the Sith Academy on Korriban, let alone achieved the rank of Darth. It's a serious title that should only be given to Sith with serious power.
