Eh, who the fuck cares about some "common sense" or "intuitive" when the game meachanica are concerned? It's all applying the game's logic and going from there.
When the devs announced the change in dot damage calculation mechanics towards what's currently known as "double-dipping", they offered a very simple in-game logic:
* anything that can scale the damage of the skill that delivers dot, can scale dot from that skill, with a few "exceptions" (or, rather, logical exclusions)
* dot can't be scaled by weapon-related mods because they scale weapon damage only
* dot can't be scaled by attack/spell/melee damage mods because dot is neither of those things
* dot can be additionally scaled by dot-specific mods that do not apply to hits (currenty: damage over time, burning damage, poison damage)
It's extremely simple really. All you have to do is to look at the damage modifier's text and ask yourself: does it refer to weapon type/weapons in general/attacks/spells? If no - then it applies to dot. Easy as that.
The upcoming system in 3.0.0 is quite a bit more complicated, but it deals away with double-dipping.
The new system will have separate bae damage values for the hit damage and dot of any skill.
These base damage values will each get modified by a somewhat stricter (than now) set of damage mods.
The trickier part now is learning the new set of restrictions as to what mods can be applied where.
Projectile damage will increase all of your damage. The basedamge of the hit will be increased, aswell as the damage of the ailments (in your case poison) caused by the hit. Thus projectile damage is superior to elemental damage increases for voltaxic rift since it double-dips onto the poison, whereas elemental damage will not.
Does this apply to Essence Drain as well?
Here's a good question to provide an example.
Is ED a projectile? Yes. Proj dmg will apply to the hit damage of ED.
Is projectile damage only related to weapons? No. Only to attacks? No. Only to spells? No.
That means that projectile damage will also increase ED's dot, as well as poison damage dealt with ED if you had a source of poison on hit that applied to ED.