My mid-layers have different functions for me and outer-layers can become mid-layers and vice versa
The first function is to bring an easy to reach and light hood that goes under the helmet when i don't want to go for an heavier hood or balaclava. MEC merino T2 hoodie do that well for me.
It also acts as a very breathable single layer after i removed my outer-layer when temperature goes above 60/65F
The third one is to bring a lighter outer-layer with wind protection when i'm too warm and need to remove my outer-layer in colder temperatures. The smartwool PHD Hyfi is my choice.