It's a geometry thing, the more you lift the rig, the more the control arm angles increase.......this increase in angle negatively effects caster, wheelbase and ride quality, as the suspension is designed for the control arms to operate on a nearly parallel plane with the road surface.
This being said, most folks can get away with stock length control arms up to 3" of lift, more than that and adjustable arms or extended length arms become very useful to both restore wheelbase and dial in caster. A lifted rig will never ride like a stock rig, due to suspension geometry changes and increased spring rates. The closer you can keep the control arm geometry to stock, the better the ride, which is the main reason behind the RE CA drop brackets and other long arm kits on the market. Others have modified LCA frame and axle brackets to improve arm angles without the loss of ground clearance that comes with drop brackets and longarms.
Also, using adjustable length UCA's and LCA's will allow one to use the same arms as they invariably increase the lift height of their rig over time, offering a good ROA.
Hope this helps.