Regular teachers
I subbed at a middle school where the teacher came in 4-5 times throughout the day, and I wasn't greatly annoyed by it because the class was on task and behaving, and he did not spend more than 1 miinute in the classroom. He did not try to "take over" the class, which I guess some subs rightly fear (since it's their class for the day). Moreover, it actually served some good because the students were having a problem understanding a formula, which he quickly resolved during 1st hour (so after that, I knew all the answers).
However, I do agree with the fact that it partially undermines the sub's authority b/c when he came in, I basically took a back seat for that minute. Lastly, at end of the day, he came in 1 minute before bell rang. When the bell rang, the students said "can we go?" He said, "ask the sub," which I suppose is a good sign because iit meant it was still "my" class for the day. As long as the permanent teacher doesn't stay in there too long or doesn't disrupt the students AND THEN LEAVE, then I don't mind it.