Well, FS ship classes are wrong anyways.
It should go like this (from smallest to biggest)
Corvette
Frigate
Destroyer
Cruiser
Battleship
Juggernaught
Cruisers like Aeolus and Mentu should be corvettes.
(Fast and maneuverable anti-fighter or anti-capship escort vessels)
Corvettes like Deimos and Sobek should be Frigates.
(Bigger than cruisers but no as fast, capable of tearing down both fighters and capships.)
Ships larger than frigates but smaller than cruisers should be designated as destroyers. Alright, perhaps this class can be used for ships that packs as much punch as cruisers but does not have fighterbay. Or packs as much punch as corvettes but has fighterbay. Or something like that...

And destroyers like Hecate and Hatshepsut should be cruisers.
(Big and sluggish, has wide variety of weapons arsenal and excellent fighter and bomber numbers. Does not have very much firepower over Destroyers howewer. Focus is on fighter and bomber numbers.)
Bigger ships than cruisers should be designated as battleships.
(Very big ships, it's a menace for all ships that are smaller than this class but does have smaller fighterbays than cruisers.)
And last juggernaughts which are the biggest ships, like Colossus and Sathanas.
(You see one of this class, RUN AND HIDE! Or be sure you have enough firepower in your pocket)
Of course I can't remember correctly real life ship classes sizes, but if I remember correctly, that is correct order of capital ship classes.
Of course balance is important, there should not be useless ship class, of course individual ships can be useless...

In my opinion of course.

So, yes there is place for frigates.
