I use them as blockers enemy path when I want to prevent the attack of a expensive unit. That not allways happens. But I use to brigade them (120points), so I use them as a single unit.
Normally they end as Bel says, in the forest, or in the swamps, doing nothing but picking their nose or hurling stones each other and keeping the moral of the greenskins high (breakpoint).
But I have to say that in some battles, when thee action is near them I always try to attack, block retreats or do something with, them... they are so fun in a narrative way, that even a great fall or obliteration of the whole unit is fun. But there are actions not planned but spontanious.