It is very diffiicult for them to pick and husk corn without the benefit of an opposable thumb.

Horses and oxen were pulling plows and wagons long before there were any tractors to do the job. If humans never used animals for farm work, most of us wouldn't be here as there wouldn't have been enough food to feed our ancestors. And as other reviewers have already pointed out, in many parts of the world people still depend on animal labor to grow their food.

Cappucino sucking, concrete-condo dwelling city slickers don't want to hear it, but this is a primary source of farm energy in a large part of the third world. So before you start bemoaning the fate of some water-buffalo in Cambodia, or an ox in Ghana, give some thought to the labour requirements of subsistence farming.

And then go order another latte and feel righteous all over. 

And if your solution is to provide machinery to do the work, tell me how that subsistance farmer affords it, or the fuel to run it, and tell me how it helps the environment.

Animals should not be used for labor that people are too lazy to do themselves.

If they're fed, sheltered, and treated humanely, I don't see anything wrong with it.

If some poor third world farmer needs a mule to plow his fields then animal labor is a life saving neccesity.

As long as the animals are treated well - that is, compensated fairly for their labor, then I don't see anything wrong with this for the most part.  By treated well I mean not over-worked, given decent shelter, Vet care, attention and good food.  Personally, I would think of them as my extended pet family.