When is it good to have four different Web search engines?

The four engines I'm referring to are Google, Yahoo Search, Bing, and Ask.com. It's good to have all these different engines in some specific situations. Some of these some specific situations are

1. When one wants to discover new Web content (using the same query)
2. When one is doing research on a particular topic
3. When one is looking for some rare information

In case (1), having multiple search engines is particularly helpful. To understand this, remember that in some situations, the sum of first SERP of each of the four engines can be more useful than the sum of the first 4 result-pages of Google (this observation, in fact, is the very premise of metasearch engines). For a query entered by a user, Google has its particular defintion of what it thinks is relevant for that query (this definition is governed by Google's algorithms). Although Google's definition serves the world well (as its market-share and its consumer-satisfaction numbers show), it's likely that in some specific situations, a user may actually be looking for alternative definitions of "relevant", something that only a different search-engine can provide. The same query entered into each of the top 4 search engines brings up results of a different type. And this different perspective is what I like/want sometimes.

Check out screenshots of the SERPs of Google, Yahoo, Bing and Ask.com for the query time pass activities. It's clear that in this situation, having four different engines helps. For similar reasons, using multiple search engines (sometimes) helps in case (2) and (3).

And that's why I'm sad at the impending folding of Yahoo Search into Bing. It was good to have Yahoo Search as an independent player...