It's easy to think that if children get good grades they are doing their homework. The reality is that many children scrape by with very poor work and scheduling habits from an early age and then, when they must tackle more complex or difficult tasks later in life, they struggle. It's important to remember that homework, while arguably tangential to the actual learning process, does offer an opportunity to practice good work habits. In fact that may be the best lesson your children can learn from all that busy work. It's a lesson I definitely wish I could have learned sooner.

Follow up:

I was a very bright student in elementary school, scoring the highest in the class on spelling and timed math tests, reading several grades above my level, always above 95 percent on standardized tests but I never saw the value in busy work. I sat in the back of the classroom buried in a book when teachers gave in- class time to do worksheets and assignments. And my teachers never objected. They thought this was just fine, a sign of intelligence or boredom common to the gifted, and maybe to some extent that was true, but I never learned how to work my way through difficult assignments. I did everything at the last minute, quickly and with little planning or thought.
Not much changed in high school. I got up at 3AM the morning the final draft was due to write my essays. I don't think I ever really thoroughly completed an AP Calculus assignment. Since my ideas were comparative more sophisticated than most of my classmates my teachers overlooked the messy disorganized packages they came wrapped in. I often made up excuses and turned things in late, incomplete, illegible and while I never got a standing ovation, I never got a good talking to either.
Now as an adult, there are no good grades to carry me and I have to organize my day and my life. The adult world offers little external reward system in the work place the way it did in elementary school. Your only gold star is a paycheck, and if you are not aware of what helps motivate you to get tasks done on a day to day basis so you can guarantee you will get that paycheck, you are in for some real trouble. I find myself procrastinating out of habit, avoiding doing things I love to do simply because I have never really been taught HOW to work. In my case, there are other serious factors that make it difficult for me to complete tasks, but I also believe that my parents and teachers could have been wonderful allies and mentors to me in navigating our time oriented world and if they had been actively involved in helping me learn how to be productive I would have escaped a significant amount of trouble in my adult life.
My parents assumed that I did my homework because I was smart and liked learning and got good grades, but in reality I did so little solid work it makes me want to cry just to think about it. All those wasted opportunities to practice skills; perseverance, attention to detail, discipline, self motivation that are so necessary to succeed in the work world. Please, I strongly encourage you, get to know your child's learning process and help them get to know how they learn as well as his/her test scores and letter grades and save them the pain and struggle I have endured.