So when teachers say to me I don't have time to teach writing every day, my response is Yes, you do, if you value it; if you don't have time, you're valuing something else more (perhaps skills in isolation or phonics drills or worksheets). Take a look at your teaching day, and decide what's really most important for helping students to become independent learners. Writing is one of the best ways I know for developing deep thinking, so I make time for it.
I especially agree that writing promotes higher level thinking and reflective learning. If we ask students to explain in writing what they have learned in math, science, and social science, they will understand more clearly what was taught and strengthen those thinking muscles. Write On!