Load bars, in effect, take weight OFF the rear wheels. Taken to the extreme, load bars can be set up to where the rear tires have ZERO weight on them, where the weight is distributed between the trailer tires and the front wheels of the truck. Ergo, best to load them properly.
Load bars should definitely NOT be used as a crutch for an incorrectly-loaded trailer or truck not designed for the task (i.e, insufficient truck rear load-handling capability). If you transfer too much load forward on the truck using load bars you'll get handling characteristics similar to not having enough tongue weight (ask me how I know...) - GA