For middle school students, homework serves to take the skills and concepts covered during class and move those skills and concepts from short term memory to long term memory. Additionally, homework, whether actually completed at home or during class, helps to instill the work ethics students will certainly need in high school and beyond.
I like to use a sports analogy to demonstrate this point: parents and students seem to have no problem with practicing the same play over and over again in a sports environment. Yet, many parents and students lament any homework that requires more than 10 minutes to complete.
Ultimately, the assigning of homework needs to pass some kind of sanity check. What's the reason for the homework assignment? Just busy work? As a way to cement material covered during class? Punishment?
Remember, many student's do not have a home environment conducive to homework. Many students must care for siblings, work, or have other duties because of home life situations.
Ultimately the reason for assigning homework is to ensure students can remember, use, apply, and synthesis the material covered during class in order to make connections to the real world. The amount of homework assigned is less important than the product of that homework. Quality verses quantity is the rule we should all consider following when it comes to the amount of homework we assign.