Why Is Joakim Soria A Reliever?

I was looking up some data today on Fangraphs and took note of a very interesting fact. Joakim Soria has a(n effective) 4 pitch mix and has recently and increasingly used a fifth pitch which is not specifically identified. Isn't there an unwritten rule somewhere that requires hurlers with a healthy arsenal of pitches to be in the starting rotation?

Interesting fact: Soria's 91 MPH fastball is complemented by a devastatingly slow 70 MPH curveball that equally as fast as Tim Wakefield's fastball.