Sometimes squires will remain on the Dropship, and assist with manning the cannons, help communications, scans or anything else the Knight Captain might require. However the Squires are still active fighters, and being planet side means they can still engage with any Plague that get through the assaults the Cavalry are engaging in. Having ground troops means more lives can be saved, and these are the times when Squires are placed there. Unoccupied worlds would not require a final line of defense.
Additionally think of how the ancients used to believe that if your brother/lover/partner was fighting alongside you, you'd fight just as hard to keep them alive.