Guinevere was a Celtic queen, so to be historically accurate she SHOULD be a warrior, because the pre-roman Celts were a matriarchal society where women held power, fought battles, and even led armies. Even post-Roman (which is around the time historians say the legend first arose), they still held on to their old traditions. Boudicca is a perfect example of this.

Did I mention this is my cultural ancestry? Yeah, quite proud of that. XDD

As for Merlin, I haven't watched it yet (plan to start tonight actually) so I cant say anything yet. I have read several books with awesome Guineveres, and Keira Knightly played her awesomely as well.