The Guard, in command or not, is required to hold out enemies from the weaker characters, more so than any other fighter class. It's in their job description.

If a Guard is incompetent, they'll let enemies in to the weak people, blink out of the way of attacking enemies and not block their advance, or 100 other ways to mess up and let skirmishers in to those who can be hurt easily.

Any other warrior can fill this role - Calum has spent a large chunk of his time IC when there was a Guard in charge defending various specific party members as a standard position - but the Guard is expected to always be doing it.