They might not have any synergy, but they do at least feel part of a whole design and not just tapped on. There is a clear resonance between all the abilities as they are intrinsically tied to whichever summon phase that role happens to be in. With dots, the only thing they really did was interact with fester and bane, but even then it was just tapped on when the rest of 5.0 class was focued on Trance and Demis.
Adding dots to current summoner, unless they were interwoven within a specific summons kit, would feel like a band aid, arguably one that wont go anywhere near to resolving the issues the class has. 6.0 summoners issue is not in its design philosophy, that in itself is really good. The issue lies in the application. The role feels overtly rigid and has little optimization opportnity. In a sense its very similiar to Black Mage, in that the rotational aspects are relatively simple. The issue is that where blackmage has a lot of scope for optimisation as and when required. Summoner, lacks those. If they can add some flexability into the role and diversify the primal GCD and Abilities to make them feel diverse enough that you arent just pressing the same button, then the class would be pretty solid. Its not another rework thats needed its small mechanical adjustments.
