She's an okay tarpit against models with low willpower. Kirai can summon her in when she needs her (like when aSeamus shows up and you have no other way of stopping his rampage).
Personally, I just liked the model. That was reason enough.
The Dire Trolls are under 3.5''. Trollkin range from just under 2'' and 3''. Pygs range from 1.5'' to 2''. Whelps are under 1''.
I never owned a Slag troll, so the average height is unknown to me.