The Hecate is admirably armed for killing fighters and deals with fightercraft threats far, far better than an Orion. However, if you need to form Battle Line and handle big targets, the Hecate is not the ship you want.
My best guesses as to why the Hecate would, or could replace the Orion is that it costs much less, and that it holds an extra four and a half squadrons. (There are twelve combat-craft in a squadron.) Also I suppose in an optimal battle scenario (if there is such a thing) the Hecate would be escorted by corvettes and cruisers.
It does work well in a surgical strike role, hitting smaller warships with its considerable forward firepower before they have much chance to fight back. But if attacked by a warship from any direction but the front, it's in real trouble and needs to either leave fast or rely on bomber wings to take care of the threat.
Oddly enough, I found this to be more vulnerable to bombers than an Orion.
It depends at which side you decide to attack the Hecate. Because it has such a large surface area, it needs many more effective turrets to cover every nook and cranny of the Hecate. At certain angles, the Hecate has enough turrets to easily destroy a lot of bombers, but at others, it has far less coverage.
I disagree with the previous posts. The Hecate is designed to be able to defend itself from all angles and not allow itself to be outflanked. While it is not as effective against other destroyers as an Orion, its turret placement means it would be able to outmaneuver other ships and attack them from angles they would not be able to return fire from.
It can be outflanked. At certain angles it is a fortress, but at others you can easily fly through light turret fire, or just disable the turret. In addition to this, the Hecate has exactly the same top speed and rotation time as the Orion. In effect, the Orion could also outmaneuver ships and attack from angles that are unguarded. I find the Hecate inferior to the Orion in almost every way.
I wouldn't criticize it for its armament so much as the placement of its armament. While the Hecate is well-armed, and can generally pepper targets approaching it from a range, it has several blind spots. Any fighter or bomber approaching from the front will face minimal anti-fighter fire and can hide in the shadow of the frontal fins and blast away, to name the most significant. The rear TerSlash is useless, because it has a very limited field of fire, and is as a weapon especially useless in the rear. In addition, the Hecate is as slow as the Orion, and has no more armour than the Orion. The forward BGreen and TerSlash, while making the destroyer relatively powerful if aimed properly, is outdone by other destroyers. I guess that the fighter bay has got to count for something but overall the Hecate is ruined potential. It could use a second BGreen to replace the frontal TerSlash, and some of the armament could be moved to different spots on the hull.