In no particular order:
It would not mean lots of extra homework time, because it will be easy for you since you're good at languages and already know Spanish (which is extremely similar).
The more languages a person knows, the more likely they'll be valuable to a company that does business with people/companies from other countries. In the ever-expanding global market, this is valuable!
You can combine your knowledge of languages with a major in business or law or just about anything else and become more highly paid than people who know only one language. (Example: My cousin knows 7 languages and has a degree in law. She has worked in the U.S. as well as other countries as a specialist in international law.)
Of course, you could become an interpreter. I still remember touring the United Nations building in NYC when I was in sixth grade. You would put these headphones to your ears and spin the dial on them to hear the speaker in any language you need. The U.N. requirement for hiring someone is three languages. Possibly other interpreter positions have the same requirement.
This way you could become an ambassador to China, Spain, OR France! (or other countries that speak those languages)
Anytime you learn ANYTHING, it expands your brain.
You enjoy languages. You will get school credit for taking a course you like. You will get good grades in it. Why not get credit and pull up your average for doing something you enjoy anyway? And even if you were just doing it for fun, what's wrong with fun? Your parents let you do other fun things, right?
When you visit countries that speak the languages, you will get SO much more out of your visit. You will understand every exhibit and, much more important, be able to actually speak to the PEOPLE to learn things you could never learn from a tourist stop.
If your parents are worried because you want to take French INSTEAD of another subject, let them know you can study that subject later on. The younger you are when you learn a language, the easier it is to learn it and to adopt the accent. Or, agree to take the other subject in school, and then learn French on your own from a computer program or CD's at home.
Bon Chance! (Good Luck!)