Hahaha.
Yoshi, I never said engage in combat while in a line formation. I specifically stated the line formation is more flexible, meaning you can change formation to anything you wish (assuming it's made of single ships).
Multiple ship formations are less flexible and less powerful in most cases. There are a number of reasons, here's a couple:
A line:
1. Gives you a larger area to watch.
2. Allows you to better protect airborne bombers. Four/six ship fleets can only expand so far and can not 'wrap' or 'bend' to provide custom fit cover.
3. Allows you to break formation and feint one way while attacking/retreating another at maximum efficiency.
4. Allows you to hug a coastline while sneaking around.
etc, etc, etc.
Want to panzer? Make a line.
Want to use guerrilla tactics? Make a line.
Want to divide and conquer? Make a line.