Considering that a death thrall is permanent (and you can have two with the twin souls perk) its hardely the same thing as the summon that you have to reacquire everytime you use it.
No, because no matter what when you find them they are always werewolves which don't count as humans. Werebears begin as humans which makes me think that if you can thrall a Werebear in his human form you can have a full time Werebear ass kicker.
Just attempted to Dead Thrall a Werebear, and let me tell you killing one before it turns into a big hairy killing machine is no laughing matter. It worked, sadly the Werebear remained in his human form.