At 675 points these guys are in a really weird place, sandwiched in between 2 units that do their job better than them.
They can't beat a single infantry unit that's more expensive than them. The best example of this are Long Beards. They are only 25 points more and Foot squires should theoretically counter them. They're an armored unit without primary armor piecing. Squires with their armor and AP damage plus their dedicated anti infantry role should theoretically win, but they don't.
You can't even really expect to get your money's worth either. Silverin Guard don't have primary AP and are designed to work against large targets, and squires cant even remove their martial prowess regardless of whether the unit is braced or not. I think the only exception are forsaken.
The only infantry they can beat are ones that are less expensive than them, and they aren't very good at that either. They'll lose to rangers, orcs (savage and civilized), and even battle pilgrims on their own roster with those units are anywhere from 75-225 points cheaper. They also struggle against units with great weapons like marauders and dwarfs warriors who will usually earn value fighting them. But why would pay for them to clear chaff? Battle pilgrims are 75 points less and perform equal or better than squires against cheap units with the only exception being corsairs (a unit that KE can also beat for only 25 points more). BP also have the benefit of ItP and more missile resistance.
At 675 points they just don't fill a role. BP are cheaper and better against chaff and Knights Errant are almost the same price and do better against elite infantry.
I'm not sure if it's because they're over priced or under powered but right they don't serve a purpose on the roster and cant even do what they're designed to do.