I think you have to add to the above the PD benefits of beam fighters. While missile fighters can be better in the right circumstances on attack, they have no defensive benefit at all, while at least half the overall value of beam fighters is on defence.
Yes... that is true... but the issue are that you can't rely on them for PD as you use them for both offence and defence. If they leave the carrier then the carrier are very vulnerable. From a resource perspective fighters are relatively expensive PD rather than using a turreted gauss. A clever opponent can wait for the fighters to leave the carrier. Having dual purpose laser cannons in turrets means you can use them for both at the same time. Improved PD, attack fighters and enemy ships. There is very little loss in efficiency.
As for missile fighters being useless that depend on how they are designed. My heavy bombers tend to get some additional 5t fire-controls of either resolution 1 or 5 sometimes both. That means they can be used against enemy fighters or missiles in last ditch efforts. So they can be to somewhat dual purpose even if not very effective. I also could potentially use bombers to intercept enemy missile fighters if I'm desperate.
I have used some dedicated beam fighter concepts in my campaigns with multiple factions and they do work if used, especially if the other side have not taken measures to combat them. Like deploying anti-fighter corvettes. One faction had 2500t anti fighter corvettes attached to their escorts with tractor beams. These where just a few thousand km/s slower than the enemy fighters but their long range weapons and heavy armour where formidable. For some reason I like to put tractor beams on larger main combat ships, they double as tugs for damaged ships or they often can bring along stations and other equipment that is needed. Perhaps not super efficient, but can be quite useful as it make the ships more useful outside their designated role.
Another development in most campaigns are anti-fighter fighters/FAC.... like really fast fighters with a miniaturized long range laser cannon or just a long range cannon. These would rush out and attack the fighters with lower ranged weapons. The side with the best engines would often be able to neutralize the other side using beam fighters but not always. It also created a race to who had the best anti-fighter escort for the beam fighters, which was an interesting development too. If I remember correctly these anti-fighter assets just tended to get bigger and bigger... the reasons for this is quite clear. Bigger ships are just better at beam combat.
In my campaigns fighters with beam weapons have mainly always eventually been relegated to interceptor duty, that is intercept enemy scouts, fighters and other lone ships or smaller groups of ships that have little or no defences against fast beam armed fighters or FAC. They also double as PD of course. These interceptors usually are well armoured and big, mostly 500t with a big long range gun and/or really fast, depends on the faction and technology at hand. But they usually are big so they can have some armour. They have to be able to take some hits. In addition to those I also have PD fighters, their role is to protect the bombers carrying missiles from enemy missiles.... these are railgun armed fighters.
So yes I deploy a large numbers of beam fighters in my campaigns too and they can be very effective in the right circumstances.
But when you come down to the nitty gritty, the fact is that the larger the ship the better and more efficient it is at beam fighting. A fighter sized craft completely waste its main purpose of stealth when used as a beam fighter. The smaller the ship the easier they are to neutralize in beam combat.
