The Squire is one of the absolute best tanking jobs in the game. As a Strength based job it gets a high amount of HP and this is only furthered by it using Leather Armor. His damage output with Knot of Rust is directly tied to his HP, and the ability can also be used to interrupt enemy channeling spells, so it works very well as a supplement to his role.
While Accumulate doesn't factor into his role at all, his other abilities all do wonders to help it. Wish can be built to be basically a one-time full recovery against any enemy that doesn't sap your MP, which can really help if your potions are still cooling down. Move Plus is a generally useful skill and allows for a quick escape in emergencies. Sentinel gives absolutely unparalleled magic damage reduction. And finally, Defend is just the icing on the cake.
If you load a Squire up with defensive gear and have a supporter at the ready, not much can break through your wall.
Tank with Interceptor Guard Edit
Building off the tank build, if you happen to get your hands on the Interceptor Guard shield, the Squire turns from a meatshield to a deadly barbed wire fence. Since damage block abilities do not prevent the damage return, it is recommended to load up the Squire with all conceivable damage blocking gear, like the Adamant Armor or the Charm Gloves. Defend is a perfect supplement to this strategy. Just from the damage return and occasional Knot of Rust you will be able to take down some incredibly powerful foes.
Since Sentinel does not affect either of your primary damage output methods, it can be used freely, and since you don't even have to attack enemies physically anymore you also won't have any problem against enemies with damage return abilities of their own, such as Adamants.