Miami had a very good line for a while, and we didn't have a back. Even with a great line, your running back still needs to have talent. There is a developing hierarchy in KC - Johnson, Holmes, and then Blaylock. This is debatable but Johnson's consistent production and big-play capability make him a better option than Priest, especially considering the age of Holmes. Blaylock isn't nearly the back that Holmes is, and it shows - even in KC.
Portis was just badly misused in Washington. They tried to turn Washington's offense into Pittsburgh's and play "wad ball." What happened was they didn't have Pittsburgh's line, and their play calling became very predictable. This is why Ricky had a sub-4 YPC year - our play calling was extremely predictable and our line wasn't good.