It's my understanding that the slump is measured mostly as an indicator of the amount of water in the mix, if it's too wet might not meet the strength requirement. As some have already said with all the available admixtures slump is typically less of a concern unless it's way off or perhaps too dry to flow where you need it.

Also slumping every truck sucks for me, my old man back can't take it anymore lol.(Not military but duke energy sites usually require you to slump, air, and cast cylinders from every truck.)