I used a knight with protect ally and played all of chapter 3 with 3 of my party members critical.
It's practically a necessity if you're gonna use BigPharma to max out your PG.
With two shields at high job levels they can all but negate almost all physical damage (and elemental depending on the shields). Templars add an attack ability that swaps defense for attack power, further increasing a 'Tank's usefulness.
3DS FC: 4356-1134-0796 // BD Chpt6 & Big Pharma Pokerus & FS: Rock ~ Barbaracle, Onix, Dweeble IGN: Enos
Knights aren't really an offensive job - the main way they deal damage is through their Super Charge ability, and for that, you want to just try to stack P. Defense. Dual Shields are usually the best way to do this, and with a decent amount of P. Defense, you should be able to reach the damage cap against most enemies. Because of Super Charge's cost, you'll want another party member keeping your Knight(s) supplied with BP, and Performers/Rejuvenation spammers are the preferred options for this. The Performer's Buff Up support ability is also helpful for Knights trying to tank enemies, since it will allow you to sustain P. Defense buffs indefinitely until the enemy dispels them or the character dies. This means that you can use Ironclad once to immediately reach +50% P. Defense for the remainder of a fight.
As a side note, any character can be used effectively in any role - by endgame, stat differences between characters only amount to a handful of points, and there are very few cases where those points make a noticeable difference in anything. Unlike many games, it's a fairly safe practice to assign job roles based on who looks best in the costume.