My teacher told me not to do homework

I honestly believe that when you are my age, you'll look back surprised at how much algebra, geometry and even trig you used. I hear your argument all the time and completely discounting my profession, I've still used a great deal more math than even I would have expected. Just the little things like figuring the mpg for gas or the cost per day/month/year of something.
What's the harm in learning something you'll never use against the problem of being unable, one day, to figure out a problem because you have absolutely no idea of how to?