I'm not too sure how useful or in demand this feature might be. It would be handy to have a couple check boxes for .5 pixels on the inside X and Y mirror.
What I found is that when you mirror you have two rows of pixels that will be the same. This happens on the far right Y and the bottom X. I think having a "Make Mirror Edge .5" check box for both X and Y could be handy so that you only have one row of pixels in the middle.
I actually ran across this when I was doing that 3D conversion. Again I am not sure how handy this feature would be. But for those looking to detail a bit more, it could be cool.