If there was an infraction type system, or something for mods to use for troubled users, not just spammers, I would agree... but there are zero tools for a mod to really do their job by controlling users. You can't punish a troubled user by marking them as a spammer...
If you setup a test account and test a special usergroup using the admin permission for managing users completely, you're going to get a rude shock at just how much comes with that permission... because suddenly a mod can now change your super admin usergroup, access permissions, etc...
That doesn't give them access to a moderator function, it gives them way too much control. I did test it with that thought... but that is going far and beyond moderator functionality when they can adjust any admin on the site when only given one permission.
Whether it be a feature, or a mod... I'm here nor there. If devs deny it as a feature, then that's ok... so atleast I know to suggest it as a mod instead.