flavor matches. Knights are designed to single out an enemy and battle it honorably.
And it's not really "suggestion-level" mind control. The enemy only has to show preference in fighting the knight, it's free to still attack other targets. I'll give you an example.
Fighting Challenge:
A choker has the party thief in his grip, and is crushing. the knight challenges the choker from across the battlefield. The choker recognizes the challenge, and fails it's will save, of course, the choker can still give that rogue one more squeeze, for good measure. Or two... or just drag the little halfling along to the fight. It's all fair. He'd prefer to be fighting the knight, but this darn rogue is just conveniently in his grip...
Suggestion:
Wizard says "Drop the rogue and come to me."
The choker hears, and drops the rogue, and comes to the wizard.
See? It's all in the details of the wording of the ability. As a GM who has played over all four of the classes from the PHBII, that's how it's best to work with it.