As far as I understand, there seems to be no logic behind this. When wanting to max things, we just need to magically know this. Just like many other additive bonuses.
There has been numerous requests over the years to make also Taeguk multiplicative.
A good and easy way is to use the d3planner https://maxroll.gg/d3planner
Take a look at the skill damage and not at the sheet damage.
For more details you can hover over the skill damage or take a look at the right sidebar under the offense tab.
Hover over damage increased by skills or damage multiplier.
https://img-9gag-fun.9cache.com/photo/aqjXwO7_700bwp.webp
Be like Jerry
Catch smart, not hard
SS used to be DIBS that actually added every primary skill bonus to character details. It was changed to be multiplicative a long time ago in 2.4.1.
i get what youre saying, my point is just to not try and give advice / correct people when you havent done your homework
You may care about this (A new hotfix to revert the nerf):
And still, nobody gives a damn about Conflag and it’s modest APoC gains. 
It really just depends on the group comp and the equipment of the DPS relative to the GR level and speed desired. Crit rating can have a lot of value, or little depending on those factors.
If one Wizard extremely out gears the other, it makes some amount of sense to ask the lesser Wizard to buff the other.
Think… Slow Time, Conflag, Frost Nova… big buffs!
Or just re-read this, it’s an error in maxroll.gg, as I explained before. The authors didn’t know exactly how FB mechanics worked. I do.
Specifically, how FB only uses your stats to scale the firebird DoT, but because Conflag is a debuff on the mob and not a buff on the casting “wizard”, FB doesn’t “see” it.
Stormcrow, also makes use of a passive skill, the same as Cain’s signature skills. Just to remind you, the random casting of spells like a fireball, are not arcane damage.
The idea of stealth, in magic deals with selection. Evocation, is detectable unless cast by a magic item, where as arcane spells rarely are.
Evocation is actually rare, because spells are cast quickly, from already knowing the location of spell components. Such as combat casting, used to reduce casting time.
Stormcrow, is a highly blessed spell caster, what they remember, plays a critical part in a celestial balance. Not a bad choice, Still arcane magic, has small risk of drawing unwanted attention.