I think you are missing the point here, although I may not have been explaining in enough depth, so here goes.
Consider the total possible design spaces for all possible ASMs, and all possible enemy vessels. There are certain design choices for you as the ASM designer that will produce better average outcomes in combat. In the specific case of having no info on enemy designs, one of those possible choices is prioritising speed, because that would allow you to better engage a larger proportion of all possible enemy designs, because the conditions for using ASMs successfully is that they can 1) catch the target, and 2) hit it. Clearly, the 2 conditions are fairly tightly bound, because both speed and AG contribute to final to-hit %age, but only speed contributes to condition 1). Having a high to-hit value is worthless if your ASM cannot in fact catch an intended target, thus one of the design considerations is how to allocate MSPs to maximise your ability to catch and hit a target of unknown performance and specs.
Therefore, one approach to ASM design is to maximise your speed. As we know, both speed and AG factor into the final to-hit calc though, so it seems likely that allocating some MSPs to AG will lead to better overall ASM performance. All missiles start with a base AG of 10. If we choose to allocate enough MSPs to increase that to 20, for instance, we use an amount of MSPs to double the AG stat and thus improve the to-hit value with a method independent of speed, but is there a better choice?
Enter the 11 AG break point. Instead of allocating MSPs sufficient to double the AG stat, we use enough to raise it to 11, getting a 10% increase for the commitment. Every subsequent commitment of the same amount of MSPs suffers from the law of diminishing returns, decreasing the benefit we gain overall. Going from 11 to 12 only nets us a gain of about 9%, rather than the 10% we got the first time, and so on until the increase from 19 to 20 is only gaining us ~5%, about half the benefit of that first commitment. Thus, setting AG to 11 is an "optimal" design choice, because it gains us the highest possible increase in overall to-hit, for the lowest investment of MSPs. Ideally, we would like to invest our MSPs in most efficient manner possible, because missiles are pretty expensive, and tie up valuable and irreplaceable resources.
Now, it may prove to be the case that there were better things we could have done with those MSPs, but to predict that we would need to have some idea of the stats of the vessels we would be shooting at, and one of the key assumptions of this design philosophy is that we don't. As I say, as soon as we have some info on enemy ships, we can start to design ASMs that counter them more efficiently, but since most games start with the player knowing little or nothing about enemy designs, it's worth considering.
Nothing in any of this places any absolute requirement to use this design philosophy, since as noted previously, you may be deliberately RPing a race that has other priorities, have other constraints like lack of certain resources or production capacity, or just decide that it isn't a design philosophy you want to use, but it remains the case that if you just don't know what is out there, using this approach is more likely to produce ASMs that are more useful against a larger range of potential targets, for the smallest investment of resources.
I'm not saying that 11 AG is a perfect solution for all situations by any means, but when you have no other info to go on, it provides a better general purpose solution than most other possible choices. Assuming that that is the kind of solution you are looking for, which it may not be... Missile design is sometimes an art as much as a science. That's why there are so many threads about it. The "perfect" solution is to design a specific hard-counter missile for every enemy ship class, but that involves other costs and problems that aren't reflected in the missile design screen, like magazine and ordnance loadout management, complex logistics, tying up finite resources in missiles that may never even see combat, and so on. Trading down to 11 AG for speed is just one approach which will probably be more beneficial for many players in many situations. Noone is going to be upset if you choose different priorities, for any reason or for none.
>> I'd argue that you never design missiles without knowing anything at all about the missiles intended targets.
Ideally, yes. If the first time you meet a given race is when their war fleet drops into Sol or a system one jump away or something, you might want some ordnance on hand though. So having some missiles would be better than having none. And having missiles that are more likely to be at least somewhat effective is generally better than having missiles specifically designed for the wrong target
>> If you design a single generic all purpose missile to "kill all stuff"
That's not what I'm saying at all. AMMs have different design constraints to ASMs for smaller targets, which have different constraints to ASMs for larger targets, which have different constraints to ASMs for long range combat, which have different constraints to ASMs for short range combat etc, etc. Having a few fire units of general purpose ordnance on hand when you need them is better than being able to design a perfect hard counter ASM too late, and getting your planets nuked before you can build any "good" missiles