There were a number of times when hiking this winter I got cool and needed something. Not cold so I didn't need my down jacket. My rain coat would have worked, but would have felt clammy when I warmed up.
My Marmott UL windbreaker worked perfectly. There are many situations where this turns out to be just the ticket. In winter when it is cold and windy it fits nicely over my light weight down jacket. Around camp a fleece under a wind jacket seem ideal, warm enough and keeps the mosquitoes out.
In a very light sprinkle may not be worth putting on a rain jacket, but the Marmott UL windbreaker resists a surprising amount of rain. (I maintain the DWR and keep it clean by washing with Nikwax TX Direct.)
I also found it ideal for attaching it (stuffs into its own pocket) to my harness when rock climbing where it may get cool up higher on the rock. It is just so light, compressible, and flexible I feel there is rarely any reason to leave it at home.
When there is a choice, always think "layering."