Do we really need a flag for this? What exactly will break if the lead scaler setting suddenly works as advertised for Hornets?
This seems like a straightforward fix of an oversight in the lead scaler feature. I'm pretty sure it's a 3.6.13/3.6.14 feature anyway, so we have no obligations of backwards compatibility to make us introduce yet another flag.
Theoretically, a mission whose difficulty is balanced partially on the basis of Swarm-type missiles (namely, the Hornet, Hornet D, Hornet#Weak, and MX-52) not leading their targets. In practice, however, I have found in my somewhat limited testing that having swarm missiles lead their target tend to increase the combat effectiveness of the missiles from almost completely useless against anything smaller than a cruiser to being effective against anything at least as big as a heavy fighter but smaller than a corvette, with a swarm missile having limited usefulness against space superiority and interceptor class fighters.
In fact, leading swarm missiles are
still largely ineffective against space superiority and interceptor class fighters due to the simple fact that swarm missiles, well, swarm rather than fly directly towards the target's lead point. Using the Hornet as an example, I estimate that with swarmers set to lead to their target, the hit rate of swarm missiles against anything smaller than a heavy fighter, outside of a head-on engagement, is roughly 10% -- 20% verses an AI opponent. In head-on engagements, the hit rate is closer to 70% -- 80%.
In summary, even with swarm missiles leading to their target, they shouldn't have too great an effect on the balance of missions...especially since enemy swarmers will lead towards you as well.
To help illustrate these changes, I've attached a couple of tbm files that use the +swarmLeadTarget sub-flag as well as a mission I designed for the explicit purpose of testing changes to swarm missile homing. I would also encourage you to test out these changes against a

mission that utilizes swarm missiles heavily, such as "Into the Lion's den" (Freespace 2, 1st Shivan mission). You will of course also need to apply my swarmer patch patch and recompile FSO to do any testing.
[attachment deleted by a ninja]