Well, see it this way:
The Plumeria has a main cannon, which can deploy a wider cone effect with its aether shock cannon. The 28 smaller turrets the Sharie has probably are the single shot version without a greater field of fire. The larger main cannon achieves what it does through something like a minute or so of charging time to hit many targets.
In a minute, a turreted cannon could attack six targets... so on squadron level warfare, this would give the main gun a ~5 attacks per minutes if facing a 5 ship squadron. The more vessels in the area of effect, the more potentially effective the weapon becomes (hence it's classification as an anti-fleet weapon).
As for the speed of the weapon, well, seeing this is pretty much a scalar wave it should go as fast as a beam... and that's 1c. The further you are, the better the chances the attack can be avoided. Unless Wes meant for it to flood the whole area at once... but previous imagery and roleplay description leads me to believe it's an attack that goes from attacker to target, rather than fill a wide area instantly.
In such circumstances, though, the weapon is still viable as a deterrent for any enemy to dare get closer, or something that can win your side some time by forcing the enemy to break formation to dodge (provided Anti-FTL conditions allow that).
If you think a bit on it, it makes sense. This is as much to destroy as it is to gain some control of the battlefield.
Additionally, it's important to note that these weapons have always been recorded as potentially inflicting abnormalities in the space they are used, perhaps making faster-than-light technology not function as well. Because of that, it is usually a sore issue to use the wide-cone more in an allied populated system that normally sees its fair share of traffic. This is one of the bigger reasons why single-shot mode on the aether shock cannons was made (not to mention the faster cycling time made it a better weapon to use in smaller skirmishes).
