Well I think what most people are referring to as "anger" in that therapists should express, is actually frustration. Anger is expressed when frustrations are not addressed correctly. For a therapist to express frustration, very understandle. For actual anger. A sentence said without thinking. Said in a loud tone or demeaning tone is inappropriate. Fortunately a therapeutic setting is not one where it belongs or needs to be at all. If they dealt with frustrations ahead of time, it wouldn't happen. Unless of course there is transference going on.
I've been yelled at by my fair share of treatment providers. Mostly because I don't talk. It seems to be that I find out who the professionals are the most easiest. I don't not talk on purpose, it's a part of anxiety that brings on a piece of selective mutism. One time upon entering a general psych ward, I was just a teenager, and I had never been there before. I wasn't a hostile teenager. I was just incredibly quiet. Without any knowledge of me I met with a psychiatrist and his resident. The resident did the talking, in which I did the usual shoulder shrug, and stared at the floor.
It didn't take him two minutes to stand up, throw my chart on the desk next to him and ask me "What's wrong with you!? DIDN'T MOMMY SPOON FEED YOU ENOUGH!?" That's when I learned about counter-transference. I continued to look at the floor as the psychiatrist stood up and dragged his resident out the door. He came back in, sat down and chuckled that I deflated his residents ego and he was sorry about that. I didn't think it was so funny back then, but now it makes me laugh. Yes I'm in a psychiatric ward because I didn't have enough barbies growing up.