I have to give some credence to what the OP suggests.
I'm now a 41 sentinel, and I have to say I was loving my sentinel. Sure, I'd have trouble with elites and have to do tricks to down em but I thought everyone did (in beta). Second beta weekend I lvled a Sage and fking pawned elites, even champions with a few tricks.
Now its release, and stupid me I roll a sentinel again. Just like the 2 sabers, call me a dork, w/e. Now I didnt feel weak, I could still down elites, but have to rest a bit inbetween. That is until lvl 35 or so. Man, the sith elites on our class quests were freaking pawning me every time.
I respeced from combat to watchman, now I pawn without resting between elites.
I think the problem is several-fold. Without repeating what's already been said about the sentinel experience (either poor skill, poor gear, poor companion gear), in my honest opinion I think the issue is your TYPE of companion and your spec. As combat, when I had my lil droid, I pawned but this was low lvls. Then I stayed with Kira because, well, she's cuter, and as combat spec, if I'm getting punched in the face, Kira's dps isnt leet enough to down em fast enough with me at the higher lvl elites (not true for lower lvls). With watchman, since I'm healing myself, with Kira we pawn elite to elite with little to no resting. Maybe if I geared up my little droid, I'd could stay combat spec and just pwn similarly. But I havent tested this and honestly, I have no motivation to do so. I'm melting faces now and am happy.
Hope that helps.
I'm not mentioning other companions at this point since I have no comparison between specs--I'm staying Watchman and that's it.