Sometimes, I use one Xyphos for their burst PD to cover Legion's rear. It is annoying that Legion does not have rear firing turrets to zap missiles, so Xyphos may be a tax to fix that weakness.
Legion is one of those ships that can benefit from Ballistic Rangefinder. Put Hellbores in the heavy mounts, and railguns/light needlers in some of the light turrets. It can beat SIM Onslaught, but it is at a disadvantage if using 900 range ballistics. If using Gauss Cannons instead, then Legion can beat Onslaught if it tries to kite with TPCs.
I had to leave several mounts empty and use cheap fighters (except for one Xyphos sometimes). I use Talon, Claw, Broadsword, and either Xyphos or second Broadsword. I also do not use missiles because I do not have enough OP to get missiles and everything else. (In case of Legion XIV, I prioritize missiles and maybe use four mining pods if short on OP.)
Sadly Legion is too big and slow to run after frigates, destroyers, and light cruisers.
That is what fighters are for, to seek and destroy the cowardly frigates and maybe destroyers than a capital cannot catch. However, frigates need to get close, and Onslaught can just gun them down with Devastators, missiles, or other guns.
And as you said, if it's not a battleship, then why does it cost as much as Onslaught?
The same question can be asked after replacing it (Legion) with Conquest. The answer is because Onslaught is strong for 40 DP, while Legion and Conquest are both weaker than Onslaught.