Google is a must for SEO (search engine optimization), that's a given. But there is a legitimate question about whether or not you should target Bing and Yahoo.
In short, no.
I'm not saying nobody uses Yahoo or Bing. In fact, it shocks me how many people actually do use them. As of February 2016, Google owned 64% of the US search market, with Microsoft a distant second at 21% and Yahoo third at 12%. This represents a slight drop in Google's share from a year earlier.
The most likely reason for the small drop in Google's market dominance in search is, believe it or not, Chrome. Chrome's meteoric rise to the top of the web browser market (depending on which statistics you believe) had a few side effects. Namely, it worried (see "pissed off") the two other major players in the browser market, Microsoft and Mozilla (Firefox).
From Microsoft, the most recent reaction was to force their new browser, Edge, down the throats of Windows 10 users as much as they possibly could. With Edge comes another heavy-handed "suggestion" of Bing for search. There's also the built in search bar in Windows that will perform a Bing search. Mozilla's reaction came last year in the form of dropping Google as the default search engine for the browser in favor of Yahoo, the only major search player without it's own web browser.
So why am I advocating you neglect 33% of the search population?
Simple. I'm not.
First, it's important to understand how the search engines differ. Google continues to get better at natural language processing; meaning they're constantly improving on ways to match context rather than keywords. In fact, most of Google's research and efforts in improving their algorithm these days is going into this area.
Bing and Yahoo are still heavily reliant on keywords, too many of which will actually now penalize your site in Google. This means if you go too far focusing on Bing and Yahoo, you can actually hurt your Google ranking, and nobody wants that.
Focusing on Google doesn't mean your actions don't affect your rankings on Bing and Yahoo. Google is the king of search for a reason; they serve the most relevant results to users.
In contrast, Bing and Yahoo are constantly striving to serve the same quality results Google is known for and always seem to be just a little behind Google in doing so. This means as long as you focus your efforts on where Google is now, the changes you're making will ultimately have a positive affect on your Bing and Yahoo rankings, if only just a little further down the line.
The other issue for most SMBs is a lack of time and resources to be able to optimize for that many search engines. If you're not at the top in Google, your time is better spent getting on the first page in Google than it is worrying about Bing and Yahoo.
One last (but incredibly important) note. Even with the changes noted above that Mozilla and Microsoft have made on their desktop platforms, the desktop is no longer the primary means of accessing information through search. In May of last year, Google confirmed that mobile now accounts for more searches than the desktop.
Seeing that Google's own Android is the most popular mobile operating system both in the US and on the planet, Google is bound to dominate that market in the foreseeable future; especially since it also remains the default search provider for the iPhone.