Someone said that melee +healer = win. Now this is not WOW. TBH Healer + Ranged > Healer + Melee. Jedi Knight and Sith Warrior need a healer to be viable...others don't and are even better with.
For Sentinals I don't think it is an issue of damage done. I think the problems are all around CC, and knockbacks. We get knocked back, we get remved from combat, and then we get dealt with. Range don't have this issue, you knck them back and they still hurt you, you bring them close they still hurt you. You give them a healer too, and they will always out perform you as you need to be in melee range to kill them.
I have a 50 sentinal. I leveled to 50 with no issues. I have owned all the PvE content bar the Emp (gearing up for that fight). I am leveling a number of ranged classes at the moment, and I am having a blast at PvP. I have noticed straight away the gulf / gap between Trooper, Bounty hunter, Sithy Sorcy to my Sentinal. Even at low levels (not 50) I have been impressed with the game play of these ranged classes. ow easy and fun PvP becomes when not playing my Sentinal. Knowing Sentinal well...I focus these guys on my ranged classes, as it is easier to rack up kills and damage done.
Very good indicator is MVP when was the last time you saw an JK or SW mvp? Rare very rare. I'd like to see the stats for the PvP for MVP votes, and damage done.