Would you really like me to explain? People have already explained, yet, nobody listens. Giving your opponent the option to draw a card is NEVER good. Honestly, do you really think playing a couple of Field Workers is going to be the straw that breaks the donkey's back? It won't! But if you think it is so good, play Field Worker because I LOVE getting free cards when I want them! I seriously don't understand people's obsession with Field Worker.
Rhyeperior might have potential, but Field Worker is just plain silly. The advantage is hardly the decking aspect of his Power since people have many ways to avoid decking. Trashing cards like that is probably the best part of him. However, the fact that he is a F St2 means you are going to have a tough time building a competitive deck.