I think it is fine, you are not using a consumable or a relic, you are simply using your abilities to their fullest. You wouldn't complain if a marauder used cloak of pain or undying rage, would you? No. And them saying they wouldn't do that in a normal warzone, that is a blatant lie. I have seen people do worse all the time
I think what shocked me the most is how other even other Shadows are getting mad at me for using it. When they use their 4 second stun on me, I don't break it, because I am worried that I will have that tactic used on me. Also I have told Marauders that I won't use that tactic on them if they don't use undying rage... they either don't have a response or they say it's not the same thing. I just don't get it. If it was really THAT cheap I think Bioware would've done something about it by now. However, it's a legit tactic that has been used since WoW days with rogues.