Er... I don't belive this, not all trolls are so stupid and incapable to do anything.
They are not cleaver, maybe not brilliant, and sure they are vicious, but not like an animal.
They are capable of speaking, make simple tools and cloaths, complete simple tasks and so on, they are more smart of an ape, but of course less than an human.
In the Hobbit, they use simple tools and swords (just as knife), the live in a cave with a secret door an so on.
Holog are more intelligent, I think Mordor can use these wicked creatures to do tasks like sentinels or trained warriors.
Just IMHO (but according to lore I suppose).