Most designers and business owners that I know and I know quite a few optimise their sites for Google organic search best they can and if they decide to buy some clicks they use Google Adwords.
Ignoring Bing however may be a mistake! - let me tell you why.
I picked my wife up from work one day last week. On the drive home as she was telling me about her day she states
"I spent half an hour Googling for a customer...actually Binging Not Googling"
I ask her
"What's wrong with Google"
Her response is quite revealing;
"The company have blocked Google"
I suspected that I knew why straight away, but I asked if she knew why and if the company blocked many websites.
"No idea why they just told us to use Bing instead. They only usually block things like Facebook and Twitter"
And there my friends I think we have the reason. Google have launched Google+ as sub-domain of the Google domains, rather than on a domain of it's own. Now if you work in an IT department and you are told to block access to Google+ it's much simpler and more effective to block all access to Google and just say to the staff;
"Use Bing instead"
Now there are over 40,000 people work for my wife's company in the UK, if more big firms decide to follow Bing searches are going to go up quite substantially.
What will Google do, well I suppose they could move Google+ to it's own domain, but have you ever had to try and get a large IT department to undo something that isn't critical! Chances are when corporations switch to Bing they wont switch back!
Now I ain't saying abandon Google, just do not ignore Bing.