Longtime readers know that I am no fan of coursebooks. However, making your own materials can be fraught with danger, usually a typo that you hadn’t caught.
Why do it though?
Books don’t fit every learner. You know your learners, the people in publishing don’t. They know learners.
Books don’t cover half of the things you want to cover. You don’t only teach tense, aspect, modals and conditionals. Adjacency (appropriate/expected responses), backchannelling, fillers, weak forms and other aspects of connected speech are rarely covered in coursebooks. You are also unlikely to cut up pages of books (though a nice resource book is a beautiful thing). Even if you don’t want to ditch your coursebook you might need to supplement it to make sure it suits your learners.
If you have something already, it’s easier to modify than a book spread. You can edit a PowerPoint on your phone these days; getting the Tippex and scissors out first thing on a Monday is a pain.
Any glaring omissions?