When using ordnance, you're limited by target locks instead of energy. Which means ion pulse missiles are almost a "free" attack in that they use no energy and do not spend the lock. Not a primary damage source but a great option for a third hard point if you are already using ordnance.
The same goes for Homing Missiles.
Ordnance Tubes plus 3 x Homing Missile hardpoints on the CR-90? Expensive yes, but compare it to Single Turbolasers...

