When it comes down to it farmville animals are not near as profitable as most of the crops you can grow on the land where you put your farmville animals. What would a farm be without animals. Farmville animals give you farm character. I think of farmville animals as decorations from which you can periodically harvest coins.
With the advent of the dairy farm and chicken coop, sudennly farmville animals look more interesting. You can fit 20 cows in a dairy barn on two small squares. Now with the dairy barn you can get 120 coins/day.
The chicken coop is even better. Suddenly the chicken becomes the ultimate farmville animal. Why? Because of mystery eggs which have special prizes in them.