Before anything, take one good look at fflogs to see the difference between the average and top dps performers between all roles:
For most of the DPS, the difference is around 10%, in flat damage terms, about 1000-1200 more dps; BLM, BRD, and DNC are the main outliers, topping out at 14% and a 1400 dps difference between average and top (BRD and DNC are entirely based on party composition. BLM tops out around 1300 and a 12.7% dps difference).
For tanks, the difference is still around 10%, the flat damage difference being 600-700.
For healers however, the damage difference between the two starts at 22% for WHM going up to 27% on SCH, the numerical difference going from 1250 to 1500. In short, despite healers having minimalist dps rotations, bringing a good healer over an average one is a greater dps gain than a good dps over an average dps job.
Given that SQEX is kind of squeamish about rocking the boat, you can see how precarious the situation is regarding healer dps. Having an involved dps rotation on top of healing duties will only widen the gap between average and top healers.
It's plainly obvious why this dps gap exists, GCD healing. Whether you misjudge the situation and need to throw out extra healing, or your shield healer needs to throw out shields to survive every raidwide, GCD healing involves sacrificing dps which ends up being a bigger penalty than performing a dps rotation poorly. Unless healers get something to make up for this fact, balance regarding healing is too volatile to give them a dps rotation. Heck, even now the most obnoxious thing you can witness in a party is a parsing healer that simply lets people die to incoming raidwides in an attempt to squeeze out more dps, which leads to a wipe if both healers have this mindset.
In order for healers to get an involved dps rotation, every healer needs a damage recapture mechanic on their GCD heal akin to WHM lilies. Call it Cleric Stance or Faith or whatever, but healers need system that boosts their damage when they cast a heal. A straightforward and generic system for all healers is best, you don't want stuff like "SGE gets more damage from Toxicosis when shields are broken" because
1) The goal is to lower the difference between top and average dps. Adding different conditionals per isn't conducive to that goal.
2) Balancing all of this would waste dev time when the goal is to have something to do when people aren't taking damage.
For example:
Casting a GCD heal gives you 5 stacks of Faith, increasing the potency of your next Single target damage spell by X or AoE damage spell by Y. (X and Y are 20% of your filler damage spells.)
So with this example, every 6th spell can be a heal without losing dps. Freecure has a purpose on WHM again, and healers can medica on every single raidwide attack without losing dps. This gives plenty of leeway for a dps rotation, and the way Faith works means you aren't pooling it to spend on heavy hitting spells and you aren't constantly spamming heals during downtime (most of the time, you'll throw out 2 heals during downtime anyways.)
Also, assume that Energy Drain is removed as it breaks this sort of system as SCH is then incentivized to only use GCD heals and spend every Aetherflow on ED.