The WDH allows greater tongue loads at the hitch ball because it reduces the bolt loads at the hitch receiver mounting points: Without a WDH, the aft bolts carry the tongue load with a pretty good lever arm, and the front ones carry zero. With a WDH there is lift at the aft receiver bolts which gets applied at the front. By spreading the tongue load better across all the mounting points, the WDH provides the ability to carry more tongue weight. The spring bars are a bonus too, in that they provide some buffer for dynamic loads on bumps.
The swing hitch is an interesting wrinkle, in that the spring bars attach at a worst case point on the trailer frame from a load perspective (the WDH more than doubles the frame load right at the bar chain attach point). But TrailManor has always recommended WDH's, and there's no reason a swing hitch shouldn't work with a WDH as long as it was built strong enough. A swing hitch bears watching, like most everything else.
It's easy to put 750# on the rear axle with a 500# tongue weight unless you use a WDH (with a 250# reduction up front) (Ballpark rule of thumb without a WDH: rear axle gains 1.5 x Tongue weight, Front axle loses .5 x Tongue weight). That can be a lot of load in most of our tow vehicles, especially when we have substantial other things to carry.
Pickup truck owner's manuals may suggest loading the WDH bars so that they pick up less (half?) of the front-unloading instead of trying to crank the bars up enough to get all of it. This might be especially important if you're worried about the swing hitch or if you have limited capacity for the extra trailer axle load. A longer truck wheelbase and a shorter trailer are factors that give you less WDH effect per extra pound it puts on the trailer axle, too.
The spreadsheets I built to estimate these things are here:
http://www.trailmanorowners.com/foru...ad.php?t=12552
3023 note: To reduce tongue weight, I moved the battery to the rear storage cabinet and replaced it with a lighter sealed battery at the same time. I intentionally travel with 1 propane tank empty as well.