Understanding the culture is important to understanding the product you provide, if it's at all different, so that can be pretty important.
Chinese love when you speak the language- but that does not unfortunately translate into the sort of tangible business advantages that many students of Mandarin hope for. It's a wonderful language, and loads of fun to learn- but the ROI is not as high as other areas of study.
Shenzhen. You're right, 10% is being generous but a lot of long time expats pick up Mandarin not at work but through the "long haired dictionary" method.
I don't know any successful business people that have been here for more than five years that feel it's essential for business. Foreign companies fail here all the time due to not understanding culture, I've never heard of any failing due to the language barrier.
Value comes from scarcity- fully bilingual people simply are not scarce in China.
Maybe just because I hang out with professionals rather than English teachers.
But I agree that general fluency is a whole lot more common than business fluency.