It would unfair to my students for me to evaluate their writing. Moreover, I never had any training in teaching writing. Such is their response when confronted with the proposition that writing should be done and evaluated in all courses. But lack of training in teaching has not disqualified them from teaching.
Writing is the very essence of academic life.
Every professor writes. Most teachers recognize good or bad writing when they see it. What they may lack are the skills of analysis and diagnosis that would help them to identify students' underlying problems as well as skills in providing correction, feedback, and guidance that will enable students to improve. If one asks students to write more, how can one conscientiously comment on that writing without an impossible increase in time spent reading and grading papers. To help students know what to look for, provide models - both of well written papers on a given topic as well as of papers with your own comments about some common problems.