Can someone give me an in-depth explanation why Summoner is arguably the worst DPS?

[ViviAnimus]

In the past I actually used Aerial Slash, but it didn't seem to actually up my DPS, maybe I was mistaken? Should only use RS when three stacks are up and preferably use two festers during that period? And please eleborate on the off gcd abilities which can be used more wisely. And yes, my gear absolutely is pretty horrible for SM

Edit: I actually used Shockwave and Aerial Slash and ended up doing less DPS than I did on the parse video. Timed my Raging to have two Festers. I usually time my raging about 20 second before contagion so I get like almost a minute of RS dots. That > 1 more fester, right?