Let's dismantle your points one by one:
>Barely benefits from crit
-Barely benefits from cast speed
That' a *benefit* - You don't need cast speed on rings, amulet, weapon, or crit. You just scale it through generic modifiers which means it's going to be *easier* to gear up. Temp chains is also a non factor for you, since it does nothing to DoT.
>Barely benefits from poison
Poison is a significant damage multiplier against single target. Up to 2-3 times more damage against single target, if you don't realize that, you don't understand how double dipping works. Poison does nothing for CA.
>-Barely benefits from leech, instead has a capped regen rate of .5% of damage deal
Regen is significant against packs. Let's assume 20k ED which is a pretty low tooltip, that's 100 life regen PER debuff, which means 1k life regen vs 10 enemies. It's trash vs single target, but it's not like you have regen or leech as CA anyway?
>-Requires the same amount of experience to level as Lightning Arrow (15249 exp for level 2, vs 70 exp for spark), yet doesn't have anywhere near the amount of power per level
Not sure why this is an issue. It scales hard end game, and is weak early to mid game, nothing surprising here. CA is even worse early game, have you ever tried leveling with CA before a 5L +2 bow?
>-Requires either an additional spell to spread the damage to other mobs or the use of LMP/GMP, which would further hinder its mid-game viability
Which is both a benefit and a drawback - you don't need to worry about scaling AoE, unlike CA where you need either AoE or +1 arrow corruption/Scion Deadeye.
>I mean, why not just use Caustic Arrow?
Base damage is much higher than CA for the same level. CA needs a +3 bow to compete. You can scale it with spell damage, something that you cannot do with CA, which restricts you to the right side of the tree. You can use a shield and dagger for Whirling Blades.