No, this is an honest question. I see two reasons to not use them. Armor is the better choice, to me at least.
Rocket Fighters: With many at 4+ speed and 12 shield penetration, shields drop fast. One can counter with tractor beams, anti-fighter missiles, and such, but that’s costly.
Only 1 frigate weapon has under 10 shield penetration: the Pulse Laser. However, only 6 (out of 11 (12 counting tribe)) have more than 20 armor penetration. At 21 armor, you have just 4 weapons.
A shield resistance of 7 or so just doesn’t cut it.