Has the Mob Equip slot ever been used for anything?

The saddle for adventurer mounts goes there.

No it doesn't.
Saddles go in the Saddle slot.

Hogs, Draco, Hyenas, etc.
Which IIRC, later get turned into skills like the rest of the mounts.

They could use that valuable space for totems. =)

All of those go in the taming mob slot.

I mean this slot that I have highlited; the slot that is occupied neither by a mount nor by a saddle

For the record I have no idea what Mob HP and MP were intended to do either.