I recently graduated college after studying Mandarin for four years, spending one summer at Middlebury College's immersion program. I would say that it is worth it from a career standpoint. Before I came to apply to teach English (it's a start), I translated my resume into Chinese. I was able to conduct interviews in Chinese. I am able to understand Taxi drivers sometimes, I can understand store adverts, radio, TV, prices, and the locals. I like to think that my ability to speak Mandarin, although not essential to teaching English itself, definitely helped me land a job. Every recruiter was highly impressed. It will help you navigate the office, and also help you not depend on others. You won't get bullshitted as much.
On another level, I like to learn the local language as much as I can whenever I travel to that place. Even if the locals speak mostly English (Denmark, for example) I try to learn the language. It helps you garner respect and at least get around a bit.
Best way to do this? Find a Chinese girlfriend, if you haven't already. That's the top motivation for learning foreign language.