I still think writers should practice using dialogue tags as little as possible because though I don't agree with everything marimo's teacher is saying, I do agree that it's used as a crutch much too often. Amateur writers often try to use it in place of actual description but it comes off as repetitive and unnecessary instead. They're meant as markers so the reader doesn't get lost and loses track of who's speaking. If the speaker is obvious and the descriptions are adequate, then they're unnecessary.
Only using the dialogue tag "said" is really freaking dumb, though. If you're going to use them at all, you at least owe it to your readers to mix it up because the words will start to blend together and take you out of the moment.