Unit study has proved to be an effective method of teaching, but doing it well involves a lot of time and preparation. With a good conventional curriculum, you can get the same results without having to go through all the hoops. You can have at your fingertips all the benefits of a conventional curriculum while reaping the joys of teaching and learning that unit study can provide.
When making your decision about whether to use a conventional curriculum, consider the value of the tools available to you within that specific curriculum. Most of us, even experienced teachers, need structure. I need a program that has been carefully designed with a complete scope and sequence. Before a curriculum even gets to an author's desk in a publishing company, researchers must write volumes of objectives and must sequence hundreds of skills to assure the right amount of exposure for each skill. I also rely on conventional programs to provide readability levels. I know I can, through trial and error, come up with books to fit the range of students I'm teaching, but I prefer books that have been written with levels in mind and then tested to check adherence to the levels. I know I can go to a library to find books suitable for children, but I usually end up using only parts of the material because a large portion is not what I want, for one reason or another.
One of the greatest tools available with a curriculum is the teacher's edition. Even if the teacher's edition has been designed for classroom use, the benefits of it far outweigh the time required to invent creative, effective lessons. As an educator, I like to be able to relax and know that someone else has made sure that what I'm teaching today matches up with good learning theory and that it flows together throughout the whole curriculum. As a parent, I like to be able to look at the goals for the day so that I can focus on the important aspects of the lesson. I like the consistency of lesson presentation. My energies are going to be directed toward enjoying my child and interacting with him rather than spending hours of research, seeking after and deciphering resources.
One of the advantages of unit study teaching is that you can teach a subject to more than one child, cutting down on your teaching time while providing a powerful learning environment. Interestingly, modifying a good conventional program is easier than constructing all the components on your own. With some subjects, the only modification necessary is to alternate grade levels each year. For example, a young child can easily be taught science from a first-, second-, or third-grade book. You may teach across even more grades by focusing on hands-on activities and putting less emphasis on the written information. You can take the same approach with history and Bible. You can also combine the teaching of many skills in math and English. In these subjects there is a deliberate repetition of skills across grade levels because students learn in a spiral. They are introduced to skills several times before those skills become their own. You can take advantage of this repetition by using the skill charts in the front of the teacher editions, grouping the skills, and teaching them to your different-aged children.
Be aware, however, that not all conventional programs are created equal. Some lend themselves more to the homeschool setting than others. Some are very theme oriented. These are aspects to investigate before making a decision. I would also want to know how biblically pervasive the program is and whether it provides delightful activities that my children will identify with.
I like the security that a good conventional program provides. I want to rely on a tried-and-true learning pace. I want the assurance that the important concepts my children need are being adequately covered. I also want to be sure that my resource information is correct. Conventional programs have been well researched and have proved effective for many years. It just makes sense to me not to reinvent all that work. I'd rather be able to relax and enjoy learning with my children.