... why can we accept dragons and magic and all the rest of it but we argue so much about what women might or might not be able to do, what roles they might have?
Because societal pressures act on women, not on dragons and magic. Both men and women should act with reference to the history of their world, their culture, and their technology. Sometimes that "with reference" means acting against the pressure and assumptions of their upbringing, but for most people -- of both sexes -- it means acting in accordance with societal norms. And if the people with power object to women doing certain things, then the risk of punishment has also to be taken into account. Plus all societies want children, it's the way for society to continue, so anything that impinges too greatly on the breeding and raising of children is likely to be given short shrift by those in power.
If a fantasy culture allows for women to be warriors, fair enough -- but where the technology is such that military prowess is predicated on strength, particularly upper body strength, then the fantasy writer has to explain how and why women are able to compete on equal terms with men. Sure women can use long bows -- but the men of the middle ages trained from childhood in using them and their skeletons reveal that their physique altered as a result, so the women must have the same training and the same deformations. Women can also use swords -- and I read of at least one "fight-book" which refers to women. But again, those things get heavy, and although skill is needed, so is physical strength. So by all means have a woman in the middle of battle wielding claymores or battle-axes, but if she isn't built like a brick outhouse, then you've got to come up with some damn good reason how she is doing it.
One of the real problems for women in the US military nowadays, apparently, is the risk of rape. Not from the enemy. From her superiors and fellow soldiers ie "I thought she wanted it" or "We were all drunk together" rape. That's with modern laws. Take it back a few centuries and think about being in the middle of several hundred testosterone-filled men, particularly after a battle when they're on a high. Again, it's not an insuperable problem for a fantasy writer, but it's not something that can be ignored in world-building, either.
I'm with Vertigo on this. Resistance fighters, fighting in self-defence (of village/family), going round after the battle and finishing off the wounded, torturing captives, leading/rallying the soldiers, engaging in feats of endurance or spying or scouting -- yep, I've no problem with women doing any of that. Put an ordinary, unmagical woman in the middle of a pre-industrial battle and holding her own for any length of time with hand weapons and I'll need a lot of convincing.