the nano is sooooooo lightweight and comfy, it is a piece of gear you will wear by itself all the time when its a little cool out, to work, around the house, town, etc. Its a simply superb middle layer, shells glide over it effortlessly.
Sleeping in it too is so comfy its downright enjoyable, like you are back in your mamas womb.
One of a few pieces that you will truly use all the time, not just for hiking, etc.
I wouldnt call it overkill, maybe the micro is, but the nano is very lightweight.