Well, yes. But see, you're missing my point. For the lightning to be effective, it has be concentrated on a target, at least for a moment or two. A target that feels pain. Durge doesn't feel pain. And yes, the impact will push him back a couple of metres or so, make him writhe, etc. But it will only be a matter of seconds before either Grievous or Assaj attack him once more, probably both. Then, Durge gets back up, and fires away. And oh yeah, Durge has arm-mounted Energy Shields capable of blocking multiple lightsaber strikes. I think he could block a few bolts of lightning before the other two distract Dooku again, leaving his shields some time to recharge.