This little fact is why I believe that silence and poison should be a combined effect. It would make both of them usable. They could keep poison at it's current power level, but have it last only 20 seconds at se lvl 4. The silence effect would also last 20 secs since it would be the same effect. It even has the same element.

Silence has such limited use that it isn't worth it either. Confusion does it better, and to top it off, the most annoying spellcaster is dark type.