I have seen that plenty of companies targeting all three search engines Google, Yahoo and Bing for website promotion.Do you really think that yahoo and msn are helpful for getting traffic on website because now a days Google is the only best search engines and now a days 90% of people like to use google for searching then why people target yahoo and bing.
different people different tastes. However personally I think google is enough. You won't miss many visitors if you don't care about yahoo.
I disagree that you should only focus on Google. Granted, it's the more popular of the three, but it makes good business sense to work on Yahoo! and Bing as well. Customers are customers, no matter where you get them from, and if you're using your website as a means to make money (which, I'm assuming we all are here), then the best business practice is to include all possible avenues of customers, whether its from the big pond (Google), or the mini ponds (Yahoo! & Bing). You need to decide however, how much of your time investment you need to make on Yahoo and Bing to convert the best return on investment. Hope this helps!