Same HP, Paladin-level AC with optimization, high Fort, high Reflex, and a class +2 bonus to Will doesn't sound low.
Their per-power damage output is lower than the other Defenders, but the Shielding Swordmage shuts down enemy damage output, the Assault Swordmage sticks to his mark like there's no tomorrow, and the Ensnarement Swordmage not only drags the opponent back to him for more damage, but also opens it up to Combat Advantage.