If a site owner wants traffic on their site - and most do - they want to make their site as easily discoverable as possible. That means being in all the search engines. Blocking one engine (or all except one) flies in the face of most business models.
The only exception I can think of is Google properties. By blocking out competitors, they can try to worsen the experience on other engines. Like imagine a video search on Bing that didn't include YouTube? But even then, it seems like someone finding a Google property via Bing still furthers their interests.
Related, "Please Don't Block Everything but Googlebot in robots.txt": http://danluu.com/googlebot-monopoly/
Odds are the more troublesome scrapers are probably ignoring robots.txt anyway, so I wonder how useful it is.
I use Bing as my default search engine, but it falls short of Google's results for a lot of programming-related queries. I still end up having to use Google a few times per day when I'm working.
That said, DDG is damn good at giving me relevant stuff on clear queries. But i find that Google and DDG alike gets into a bind if i get a bit technical.
There is also startpage.com (!s(tartpage) in DDG) to give anonymized Google results. And also ixquick (!ix(quick), natch) for a meta-search (not sure exactly what engines they cover).