Leviathan's tempering seems more like a punishment than a method of creating slaves and securing prayers. Sastasha HM made it seem as though Levi tempered those pirates because they failed him, turning them into the tortured and grotesque "drowned men." Also, it's noteworthy that he only seems to temper men (as in mankind, not the gender). I don't think we've ever seen an example of a beast tribe member being tempered, so it seems like something he exclusively reserves for those who have failed him or he wants to punish.