Final Fire defense is very close range, and I believe that's what CIWS is made to be. While your normal laser is firing from surface to orbit on the order of hundreds of miles (or hundred thousand km if it's a good laser) your CIWS is more like a mile range, when you can get a visual feed on the incoming missile and peg it at handheld weapon range.
And yes, engineering brigades are 5x the size (including use for PDC barracks) but they also add a considerable amount of construction power, allowing them to perform certain industry tasks, like assembling a PDC from prefab. I design my main troop ships with 5 bays, enough to fit a brigade HQ and the 4 underling squads, or to carry a single engineer. Larger warpath troopships get more like 15-20, enough to fit an entire division (11 slots) plus some replacement battalions and some marine companies/battalions. Replacements rock as they basically give up part of their readiness to replenish another troop's readiness, adding new troops and equipment, extremely useful mid-fight and after-fight. Dropping a whole brigade with an additional replacement will give you surprisingly impressive results. As the enemy strikes you, you replenish just as fast, and you're still firing at full power while they're whittling down. And your highly trained troops have a chance of surviving and retaining their training and/or commander bonus.
I've also heard of CIWS being used to absolutely shred enemies. If you can get your ship in close enough, then you can rip into an enemy with your PD weapons.
The wiki states that CIWS is purely defensive.. so I'm not sure thats accurate. Although OTHER PD weapons (like my gauss cannons), I can certainly see ripping enemies to shreds with.
I knew thats what engineers and replacement battalions did, I just wasn't sure how large the former was. However, I never thought of using replacement battalions during combat.. I really like that idea, and I shall be using that from now own.
Currently, I'm designing my initial fleet around four Destroyer-sized (3000 ton) ship classes. This is just the initial fleet though, and I honestly have no idea how well it'll fare.
One ship is the main assault ship and it bears one full sized gauss cannon and three smaller 33% accuracy ones all using the same FC, along with two CIWS's and a small resolution active sensor.
Edit: Modified this ship and scrapped the three smaller 33% accuracy gauss cannons in place of another full sized one. Same dps when taking into consideration accuracy and number of shots.Second ship is the dedicated escort, which uses one CIWS, six 33% accuracy single gauss turrets with two fire controls and the same anti-missile active sensor.
Third ship is the 'Leader', and it has 2 CIWS, twice as much armor, a larger active sensor, and thermal / em sensors.
Fourth ship (I'm still designing it) will be a small combat troop transport. Fourth ship, the transport, can hold four marine companies to be combat dropped into ships or onto the planet. Has 1 CIWS, 5 layers of armor, and can move up to 5000 km/s.
All four of these ships are 3000 tons and move at 3000 km/s
and are jump capable. (Forgot to put the jump engines on the ships. Designing a dedicated jump destroyer to make up for it. DURP) The combat troop transport might be faster so I can have better accuracy with boarding, though. None of these ships are extremely long ranged, generally prefering to stay within one-two systems of a maintenance/refueling facility. All CIWS's shoot 8 shots per 5 seconds, and all gauss cannons shoot at 4 shots per 5 with a 30km range.
I don't plan on EVER using missiles.. gonna rely entirely on close range and boarding.