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Ahh...er...I understand how each works and can make plenty of assumptions, but I think it would be better for an expert to try ans answer your question.
For the most part, I don't see them throwing away efficiency, beyond the additional rotating mass, except for when they're engaged...and 99% of the time they aren't. There may be a slight efficiency hit for the viscous, but I doubt it would be noticeable.
As to the helical....yes, it needs traction to engage, otherwise there's not enough torque to spread the gears/clutches/whathaveyou apart and cause them to lock up. If you're on ice or with a wheel off the ground, you aren't going to be able to apply any torque because said wheel will spin freely.
The point of them is on the street or track, you will almost always have some traction, so they still do their job. Even if one breaks loose, it still takes torque to keep spinning a tire on pavement, and chances are the diff will have already locked up before you do.
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