It seems Bing has been quietly growing into a real competitor to Google. $1B contribution to earnings is a massive step forward. 

Being the default search engine on MS devices is a big help; there are a huge number of Windows users that never stray beyond their defaults. Obvious pockets of users would include those that do not necessarily "get" technology and those that have their PCs locked down at a company level. 

This means that Bing can be a better choice for B2B marketing if your audience fits this description. Certainly I can think of many lawyers and accountants that do.