Going into battle completely covered head to toe in plate armor is not sensible in most cases it really makes much more sense to be able to breathe and move freely. If you want to cover your female in heavy armor because you are worried about showing skin and being sixist than do it but don't fault the concept art, it's only concept art. If you want to talk about being sensible complain about the armor on the Orismer in a desert covered in plate armor not the Redgard female who is actually sensibly dressed.
The image mentioned: http://cdn.medialib.computerandvideogames.com/screens/screenshot_282252_thumb_wide940.jpg
A prime example for what this thread is about, by the way. The orc and the woman in the middle are both wearing the same set of armor, yet the woman sees fit to expose the part of her body offering the shortest way to her heart.
As for the "forsaking protection in favor of agility and thus doesn't need protextion" argument that is boudn to come up:
A bare midriff doesn’t make anyone more agile, neither does a boob window.
How many women do you see day-to-day wearing such exposing clothing? They choose to dress that way because they feel they are save in their environment. If you’d go to battle, it would only be natural to cover up as good as you can, alone for the psychologc impression, even if it’s only cloth. Except maybe if you live in a society where almost-nakedness is normal, which in most games you don’t.
If you want to use your agility to substitute for protection, you better be very agile. How agile? Well, how much agility is enough to survive a hail of arrows unscathed? Or evade enemy strikes when you are surounded by toher fighters and barely having place to move at all, as is the case in most great battles? If you only count on your agility, you have to be aware of everything happening around you and you have to react with almost preternatural speed. One single hit could end you and if you fail to see or react in time, you are done. In the middle of a battle with all that fighting going on, missing something isn’t a question of if, but when.
With armor that only covers what is indecent so expose in public, you have no protection when that happens. Sacrificing a bit of agility for a full suit of hardened leather or chain mail (hell, even full plate armor isn’t as encumbering as it looks like. You could do gymnastics in one) is the only reasonable thing to do, unless you are some sort of super ninja. But then you wouldn’t run around playing cave explorer anyway.
Agility is the go-to argument to justify porm armor but it is utterly wrong.