I also think it's because of pet positioning and the pet soaking up every hit over and over. Pet's ignore some AoE types, while others they take full damage from and get one shotted, which makes things a pain sometimes. But I too have noticed arguments over shout where people call out summoner on being too weak and such. when I played summoner, it felt fine, they are just a little slow at starting up as they need to get DoT's up. But once the damage over train is rolling, things are generally great. Plus the damage over times does damage while you're dodging AoE's which most people don't even have time to do, as well as your pet doing that as well.

So there are certain advantages to summoner, it's just in certain fights I think their downsides go against them. I'm not sure, but I think in the titan fight, pets might be able to be ringed out? Which I could see causing a massive problem for a summoners damage.