Purely aesthetical reasons: those large-scale battles would've looked very silly if all Ships were standing still shooting at Shield bubbles for 5 minutes before one dropped down.
To have movement, you need to mow down some of the Ships pretty fast, so the Defiant can carve up foes with its forward-facing weapons while manoeuvering around.
Slow battles with Shields are fine when there are few Ships and the story requires time to ponder options, but for large-scale battles it wouldn't look right.