ABSOLUTELY! PLAY LOTS OF SPORTS
Having been involved with youth sports since 1990, I have concluded that it benefits the child to add other sports, to a primary passion and game that a child prefers to play all the the time, whenever possible. Actually, when we do private sessions, we train both parent(s) and child, so that it promotes putting aside a scheduled play and practice time for kicking around the ball. Sometimes inside the home when it is raining outside. It makes for fun exercise and quality time that you will remember forever. When inside, use a soft window proof ball!!
SHORT ANSWER - The best reason for having your child or teenager playing other sports today, would be to help in the development of their body, muscles, footwork, coordination, strength, speed, timing, dexterity and stamina/cardio. All factors that he/she can benefit from in soccer and its unpredictable demands physically, mentally and emotionally. Especially, during competitive training and games.
I started playing soccer around 5 or 6, mostly to be around the big kids in the neighborhood. Sports was not a primary focus growing up in The Hague, Holland, aka The Netherlands. There, we grew up playing in the streets, parking lots, sidewalks, or other flat areas, plus the beach. We also played on patches of grass next to the city trams tracks. Real soccer fields were never open or available to the public.
Aside from soccer we needed to learn how to swim, because Holland is a country that is mostly below sea level and we were always in danger of floods.
It wasn't until I started living that in California that other sports became an important part of my own soccer training regiment. First baseball, then martial arts, followed by badminton, basketball, cycling, wrestling, fencing, hiking, running cross-country, and tennis. A lot of tennis, probably because the SF Bay Area, at least the peninsula, had the most tennis courts per square mile with lights and the best weather to play in, well into the night. We used to play for hours. But that was a long time ago, before video games dominated how kids spent their time.
Bottom-line, introducing other activities and sports, to help with your child's physical development, is a very good idea so that he/she does not develop problems from repetitive movements in just one sport.
So, lead by example! Everyone will love it. Just make sure that you as a parent allow for enough warm-up time and stretching. You will need it!