Personally, I would absolutely agree, if
we are talking about a quality coil and system such as the Icon leveling kit.
There are however, lesser quality coils on the market that could negatively impact the ride quality and performance of a truck.
Some people desire to maintain their stock handling characteristics while gaining a couple inches. For these people a spacer may be the "best" option. Others may not have $1,000+ to spend on a leveling kit. For these people too a spacer may be the "best" option.
These things being said, to your point I will concede that if
we are talking about a quality coil, replacement springs will provide the best performance from the driveway to the end of the trail and back.
Regardless, an adjustable track bar (or drop bracket), castor shims, and new shocks are, if not necessities, huge improvements to any kit; and you are 100% correct, in this business you always get what you pay for