I'm wondering why it is not possible to mirror sketches, would it require a considerable effort to allow the mirror feature to do this?

It would be handy for models that are symmetric and smooth across the centerline.

Sort of like: http://moi3d.com/forum/index.php?webtag=MOI&msg=1398.19

Of course, building half is easy with the "normal to sketch" constraints in loft and boundary, and if the curves lie planar you can mirror them in the sketch. But I think it would bring another method to the table, depending on your model and a person's workflow, that adds flexibility.