These AI formations appear to be different from the art "formations" controlling how units display on a tile.
Right, sorry, got a wire crossed there, scratch that part about damage states. (This is what happens when I post from work.) This formation required logic is separate, dealing with whether an AI uses that particular logic algorithm; I have tweaked a few of those over time (mainly to get the AI to use its future-era units in my mod better), but I invariably end up going back to the baseline once something breaks with the AI. As you can guess, a LOT of stuff breaks when you mess with the AI.
In this particular case, I'd have to wonder: if you force the AI to always escort every settler, what does that do at the start of a game (when you'll usually send your military units out to scout while your first settler plants himself, or even your second settler)? Does it slow down the AIs too much, to where the human player will find it too easy to get the ruins? And if it only classifies units with the "Defense" AI as the best choice for escorts, how many of the ancient-era units would qualify for that? I mean, back in Civ4 it made sense since every city needed to be garrisoned, so you'd send a defensive unit along, but a newly planted city in Civ5 can defend itself pretty well in the early eras. Having to drag your scouting units back home just to spend five or six turns walking each Settler to its site is kind of a waste; it's only once you get a road network in place that this gets more manageable. (And even there, it's skewed against the AI, who won't build roads towards a settlement site, only between existing cities.)
If you don't want it to be quite so absolute, then you could just add another military unit (maybe even one with the Attack AI instead of only Defense/Counter) to the formation, and keep both military units non-required. Presumably this'd just increase the odds the AI sends something with the settler, even if you made it favor a defensive unit. And it might open up some options; if it sent an Archer as an escort for your second city then it might have a harder time dealing with Barbarians along the way, but it'd be a much better complement once the city was founded... Even if you don't want to go the Attack route, just making a second (or even third) military unit with similar AI settings should just encourage the AI to protect its Settlers better. You might still get an unescorted one here and there, but you'd be just as likely to see two or three units screening in. So if the first unit is Defense/Counter (which basically means spearmen, pikemen, etc., plus the occasional Warrior/Swordsman) then add a second one that's Defense/Attack, a third that's Defense/Fast Attack, etc.; it'd still prefer using defensive units for this, but it'd now see mounted or archer units as acceptable substitutes if there isn't a melee escort around.
For a scenario I'd say go with the required flag, but for general play it might cause too many issues in the early eras. You should experiment with it before dropping the idea, of course.
