A few uses of Lammashtas (and Horrors, that other non-friendly summon):
- An immortal leader in your own dominion can be scripted to cast a few of these. They will continue the good fight after he is dead, and who cares about dying in such circumstances?
- As Chazar mentioned, Wrathful skies plus something to keep prolonging the battle can work. With a bit of luck, your Lammashta caster might even survive to retreat (or he might not). Of course, if you sent several and each was affected by the ritual of returning...
- In practise, they are mainly of use as a sort of assassin rather than part of the main battle line. However, there are variations. You could try to have some mages scripted to Summon Lammashta and others to Control Undead (haven't tried it, but it should work - not sure if it is worthwhile). Or you could bring along somebody to horror mark the enemy's leaders and hope that the Lammashtas would prefer killing off those nasty horror marked people (once they have chopped up the spell caster, that is). Cursing and Horror marking good enemy leaders is fun.
Quite probably there are other uses I have not thought of.