First, a skill/traits that helps generating Addesting. Toxicon is Sage's mobility tool, I love using it in dungeon pull but not while healing EX.
It's cool to generate 3 when there is downtime but having to shield 3 people is not fun.

As for the Addersting skills themselves, if the previous request is fulfilled, then I'd like a GCD on a 60s cooldown.
It might just be me, but 60s cooldowns used asap helps tracking the 60s party buff.

IMO, Addesting is and should be only tied to damage, otherwise you'd see many conflict about people being accused of griefing by not using their addersting to heal.
If like Assize it's a primarely DPS skill with a bit of healing attached to it, I don't think it would be trouble.