Even if the dark trooper doesn't kill that much, strategically he's the most useful because he can get to far off CPs faster. If theres a couple of guards he can deal with them easy enough with grenades and a couple of shots, and then capture a command post behind the lines. This is really useful if you only have 1 command post left and need to break out fast. I actually prefer him to the Jet Trooper, because his jump pack is so much faster- it takes a lot of luck to hit a flying dark trooper.
'If you're the last man standing you're not fighting hard enough!'