@Thirteen
There's a few problems with that:
1) I can only add alpha maps I create or the developer allows me to.
2) Alpha maps are specific to the mesh. I don't think it would be very useful unless you happen to be working on the exact mesh the alpha maps is for. Except hairs, those I could see it being useful for.
3) Alpha maps are completely out of scope for this program as they only come in 2 colors (black and white), there's no need for multiple shades.