What is meant by a "commercial" middle grade novel? My guess is something that might have the potential to become a movie or series (ala Harry Potter or Traveling Pants).I take "commercial" to mean that a book will likely appeal to a wide variety of kids in its age group. This usually increases a book's likelihood of becoming a series, and because kids like series (and adults, too, of course), the publisher will happily expand on any work that is popular.
Let's not take the easy route and hypothesize that more popular books are less worthwhile. Certainly they tend to be less literary, but they're just as hard to write as less commercial books, and when it comes to getting kids to read, I don't care which type lights their fire.