Interesting to see this pop up, I've recently been looking into it and in the process of trying to convince my agency to at least consider Bing when making recommendations.
My opinion: If comscore is correct, Bing + Yahoo = 29% of the search market. Net market share and statCounter each have them at 14.2% and 18.78% so obviously theres some sort of disparity here but you can kind of assume Bing is somewhere around 20% of the search market.
I don't know about you but I wouldn't ignore 20% of my users. And if you're site doesn't have that much traffic from Bing that doesn't mean it's not worth it, that likely means it's a huge opportunity because you should be getting that traffic from Bing.
Not only that, not everyone optimizes for Bing so it should be pretty low competition. You're not going to be fighting with a ton of SEO's for every keyword etc.
Obviously, optimize for Google first but if you can somehow meld your strategies to service Bing as well (yeah, don't know how to optimize for Bing, just know it's probably a good idea) it would probably be worth your time. Of course this is different for everyone. Bing has a different user base (Im pretty sure it's the same people who don't know what a browser is and click "that blue 'E' thing that makes internet' and have Bing by default on their computers)