Pixel perfect mode would allow to move bones only to Global pixel perfect positions while using the translate tool. Keep in mind that most people that would use this mode would rarely use rotations in setup mode, as this is meant to be useful for working in pixel art or in really precise stuff. It would save a whole lot of time since setting each image and then adjusting the value using the keyboard takes up a lot of time and kind of break the workflow when you are animating something that only uses translations.
The problem is that this would not mean truncating the global position eachtime you use the translation tool on a specific bone or attachment, since the pixel perfect position of an attachment is not always a X.0 type of coordinate, sometimes (for odd-sized attatchments) is X.5, and that precisely is what breaks the workflow when you are trying to get perfectly adjusted images in Spine.
So, if you have a even-sized attachment it would mean that your global attachment position would look something like (X, Y), and it would work fine if you truncate the position of the bone that contains the slot with the attachment each time you use the translation tool on it.
But if you have a oddpair sized attachment, your global attachment position would look something like (X.5, Y.5), but once you have a adjusted pixel perfect in setup mode for the attachment truncating the position of the bone each time you translate it would work just fine.
It's just a way of saving a lot of time for those who want to use spine for animating pixel-art stuff,. It is nothing that can't be made right now with spine by spending some time typing and being really REALLY careful not to screw up the position of your attachments.
At runtime I believe it would only be a matter of disabling filtering for each texture and using low resolution to match the retro-feel so there's no problem there.
