I remember back in the olden days when I was just starting out as a teacher, I felt like I NEEDED a textbook, because without it, what could I possibly do in class to fill the time? It just seemed so complicated and stressful to have to figure out what the students needed and were interested in and then design my own materials. But, as I've become a more experienced teacher, the less I've relied upon textbooks and if given a choice, there's probably a 50-50 chance of whether I'll use a textbook for a given class or not.
I'll generally use a textbook in the following situations:
1. I'm inexperienced in teaching the subject matter. This semester, I was given 3 sections of advanced academic writing. Although I've taught writing before, I've never gone this in depth with it.
2. It is a content based class. For example, if I were teaching an intro to psychology class in English, I for sure would use a textbook of some kind.
3. If forced to. Most unis in Korea have some sort of mandatory textbook that they require you to at least pretend to use.
4. It's an exam preparation class, such as TOIEC speaking or something of that sort. It's just too time consuming to come up with enough practice questions on my own without this.
If given a choice, I'll never use a textbook in the following situations:
1. A conversation class. I find it much more interesting and useful to just design lessons around themes of some sort. Structural based syllabi, upon which many textbooks are based are bad news in my opinion.
2. Advanced level students. Authentic materials (real newspaper articles or youtube videos for example) are much better for these students than inauthentic stuff from a textbook.
3. A current events or contemporary issue in society kind of class. How can you use a textbook for this? It's far more interesting to talk about stuff that's actually in the news today.
For more details about this topic, check out Teaching Unplugged by Scott Thornbury, which is the first (and only?) book to deal comprehensively with Dogme ELT.