-sigh- That's exactly the point. They [i]don't[/i] fight alone, they fight [i]together[/i]. That's how every single Meister/Weapon pair in the series fights. Together.

Being a Demon Weapon does not automatically mean you cannot be a Meister, and vice versa. They are two separate things, with two separate roles. Being an [i]autonomous[/i] Weapon, on the other hand, requires entirely outstanding power, and being an autonomous Meister requires the same.

EDIT: This is why Maka's autonomous Weapon-ness at the end of the anime is rather disappointing. She hadn't earned that, there wasn't any build up to it, nothing. It was just pulled out of thin air, and because the manga went on without any mention of it, it seems ridiculous in hindsight.