As long as there's a difference between the major search engines, there's at least some sort of reason to go with one of the minor engines, for some fraction of people, some fraction of the time. If all the search engines are the same, Google wins because no one has a reason to switch and Google has the advantage of inertia. It is possible that it's better for Google to be identical to Bing rather than superior to Bing, especially depending on how situational or subjective that "superior" happens to be.
While I'm not a huge fan of Google, I don't think they would intentionally have a dick-ish strategy like that. However, I do think that their test-driven nature could push them in this direction anyways even if they don't intend to. Every time they try a feature and people don't like it, they can switch to Bing and Google gets the feedback... unless it's a clone of a Bing feature, in which case people can't really switch. So bad ideas that are clones of Bing features have a statistical advantage.
This is not to say that cloning Bing features is always a bad thing. I actually prefer Bing. I'm just saying that clones of Bing features have a statistical advantage during testing, regardless of how good they are in an absolute sense.
The solution, obviously, is Duck Duck Go. Or, more generally, if the search landscape is less of a duopoly, this phenomenon wouldn't exist as much.