We'd covered this territory in the pre-hack forums pretty thoroughly, but since it's gone...
Unibody construction is stamped steel to give directional torsional regidity. The forces a unibody are expected to handle are accounted for in the directional stamping. A WDH may stress a unibody in directions it wasn't designed for.
A WDH also distributes (at least in theory) some of the weight
of the tow vehicle back across the trailer's axle. It would not be a good idea then, for example, for me to try using a WDH with my 7000 lb Excursion and put 500 lbs additional weight
from the tow vehicle back onto my Scamp's axle.
When the tow vehicle and the trailer are fairly equally matched weight-wise, and all of the compontents are capable of accepting the weight and the hitch components, a WDH may make a huge difference in the capability and handling of the entire unit when assembled. The Honda Odyessy, I believe someone has said, requires a WDH for tongue weights over a certain number, or for a towed load over a certain weight.
The Reese Dual Cam with 600 lb bars was an ideal setup with my Toyota Compact truck and my Burro
17'. It dealt with the tongue weight as well as potential sway issues and was solid as a rock. I wouldn't consider using WDH towing a FGRV with my Excursion as it's neither necessary nor appropriate. I wouldn't think of towing my Airstream 34' with the Excursion without WDH/sway control. I am currently not using either WDH or sway control with my Tundra/Scamp 16 combo and so far it's doing fine.
The use of both WDH and sway control are entirely rig-dependent. One trailer may tow wonderfully behind a specific tow vehicle, and the same trailer could be so squirrelly behind another as to make it a white knuckle drive every time you're out.
Sway can be induced by a number of factors, some of which involve inherent design flaws in the tow vehicle that are entirely independent of the trailer. Some have to do with tires
, some with tongue weight, and some with trailer loading and balance. Some can be overcome by judicious loading and attention to tire pressures. Some must be addressed with WDH and sway control in some tow vehicle/trailer combinations.
There are some vehicles that should just not be used as tow vehicles, particularly lightweight 4WDs with short wheelbases (and I've done it...), but if you must use a marginal tow vehicle, then give yourself every advantage possible (without damaging either the tow vehicle or the trailer.) Know your options. Know the limitations of your tow vehicle. Load your trailer appropriately and balanced. Employ those aids that you need, but don't overburden either the tow vehicle or the trailer with stuff you don't.