This is a bit surprising to me. I've read through many guides the past few days and all of them heavily use SS. What am I missing?
Why are you saying DS > SS? It's not like you have to choose one or the other. The spell rotation will look something like this with DS thrown in when I can.
IT > PS > Pest > BS > SS
SS > SS > SS
Pest > BS > SS > SS
SS > SS > SS
This is with the Glyph of Disease.
Alternatively, although a bit more complex, it could go something like this.
IT > PS > BS > Pest > SS
SS > SS > SS
SS > BS > SS > BS
IT > PS > SS > SS
SS > BS > SS > BS
SS > SS > SS
IT > PS > BS > BS > SS
SS > SS > SS
This would be with the Glyph of Scourge Strike which will generate more threat.