Imagine for a moment that you want to go for the Militia line in feudal age. You get Supplies and start producing MAA, and you get to your enemy’s base, and you’re doing decent damage.
Then they make an archer. This archer moves faster than you, so with micro, you can never catch up to them. You also cannot get away from them. This means that, while they are accumulating an army, you are only losing units.
So naturally, you stop making MAA. But if you stop making MAA, you don’t need Supplies, which means Supplies may as well not be available until the Castle Age.
Now you get to the castle age. Because you stopped making MAA, you have no troops for an immediate power spike. Still, you want to make Longswords, but wait! You need to research Supplies first, AND the Longsword upgrade, AND squires(so the exact same thing doesn’t happen there, too!), AND probably gambesons. That’s a combined 125 seconds of research time, which means the other player, who is producing knights, has 4 more knights out than you.
To win against knights, you need about a two-to-one ratio, plus he has a bonus 4 knights from the research time he’s saved, so just to break even with him on two stables, you need more than four Barracks going!
Wow, this is really starting to look like an all-in strategy. Now, if he has any ranged options at all, he can build 1-2 archery ranges and still micro you down like nobody’s business, while his knights wreak havoc in your economy. If you try to go for skirmishers, his knights will now trade better than evenly with you, so that’s not an option either.
It makes perfect sense why going for this is not a very good strategy.
But consider if you can get squires in feudal age?
It starts the same; you make a few MAA and harass them. But now, you have the option of Squires being available, so your whole approach to the game can be different. You know if they make archers, you can run. Yes, they can still chase you away, but it will be more akin to spearmen chasing off scouts than villagers killing deer.
So you grab supplies and build half a dozen more, and continue production. Naturally, they respond with archers, but now you can justify Squires, too. Now, when they move to chase you off, you can just send your MAA back, and actually escape! You can attack from multiple directions and move their attention around constantly. They are forced to focus their archers in one place to achieve critical mass, while you can poke at them from multiple directions, taking advantage of their lower micro needs and high building damage to force your enemy to respond. Focused on defending themselves, they will be far less capable of attacking in turn.
You reach Castle Age with half a dozen MAA already on the field, near their base. You grab the Longsword upgrade, and when their first knights come out, you already have a numbers advantage, so once again they can’t engage! Just by continuing your production, you can maintain this advantage for a significant way into castle age with just the barracks you already have, and instead of being forced into more barracks to force your way to a numbers advantage, you can build a monastery or siege workshop or archery range to deal with their archers.
What was a very unstable and negative position is now a reasonable and potent position, just by allowing squires to be researched in the feudal age!
To sum up.
MAA chased down in Feudal
MAA die/never produced
Supplies never researched
Numbers disadvantage in castle
Too many upgrades
Can’t get enough production
Lose to knights
With squires in feudal
MAA can escape in feudal
Supplies researched in feudal
More MAA produced
Squires researched when archers come out
MAA stay alive to castle age
Reach castle with numbers advantage
And that’s why Squires should be available in Feudal Age.