while it will always push up the level of the servitor different tiers use different skill sets and it often having access to different servitor abilities vastly overshadows the boost to their ability levels.

for instance luminous archons heal half as much per heal casting regardless of its level relative to other archons.

abishai devils spell list is skewed towards direct damage vs maledictions and it has a lot fewer choices for both and weaker AI in terms of when to use which then other devils.