Look, y'all, these things are actually good.
Here's how it throws down.
The YV-666 has a 180* arc, but you may be finding that its easier to catch people with the extremity of it rather than the front.
This is due to how template-based maneuvering works
Basically, as you move forward to some degree with every maneuver in the game save the "Stop", your target window is compressed in front of you, while stretched behind you, when charting over multiple rounds.
If you start charting the angles you fire with Turreted ships, you'll see this play out.
Additionally, clashes in this game tend to adopt a swirled structure as both parties attempt to keep a bead on the other.
Using these two mechanics together, you'll find that within a specific match, you'll be Turning and Banking in one direction far more than any other combination of moves within that match.
With Maneuvering Fins, if you're expecting a Bank to be superior, select its Turn instead. Your effective firing arc for that round is the combination of the two possible arcs you select, assuming you have either an Intelligence Agent or a Pilot Skill advantage.
That effective area is massive, and also allows a minor amount of arc-dodging.
Its clear cost is an opportunity cost: You can't have MF with EU.
However, EU is 3 points more expensive, and costs an Action to activate, and simply keeps carrying you forward rather than adjusting your arc laterally.
For that 3 point difference? Equip a Navigator, and simultaneously select all Bank and Turn maneuvers on that side of your ship, without ever getting stopped through action-denial.