For example, your flutes and clarinets will learn 8th notes, triplets and 16th much more quickly in concert music than your tubas. In a method book, everyone is being introduced to the same concepts and you can adjust the tempo to meet the needs of the class.
Yes, you can teach those concepts on your own or create your own materials, but the book has them already organized in a well-thought out progression. That way you know your students are getting all the skills they need.
I can't speak for other methods, but EE Book 2 does a really nice job of introducing a concept and following through on that with rhythm counting and songs that fit the concept.
Just my take. There is certainly more than one way to skin a cat.