It's the most common Pokemon to see in shaking grass. However you don't see shaking grass often which actually makes it rare.
The reason you see it so often is partly due to the fact that you need a Pokemon in shaking grass that you can't find anywhere else. Gamefreak needed a Pokemon that would be the Patrat of Shaking grass. This way you don't find the REALLY rare Pokemon so easily.
They also give a huge amount of experience.