Had one while I was leveling that didn't even have anything in the off-hand slot.
Just to be nice, and to save myself some work (as I was the Healer), I had one that I had just finished for my tank companion, who used the same stats as him, so I traded it to him and he still didn't equip it.
Well, that could be because you couldn't see his off-hand. I've noticed a few times that when I inspect people (say, a tank) they don't have an off-hand equipped. But then when I ask them about it, they reply that they do have something, and link it in the chat.
Even had Marauders seemingly with only one weapon equipped, judging by their character screen, but who obviously had an off-hand saber (as I saw they swinging two lightsabers). Swinging two lightsabers while only having one equipped is a feat.
So that could've just been a bug.
I have also seen my fair share of bad tanks/dps/healers. Been a bad tank once, myself, but only for the first few trash mobs before I learned how to tank better.