I'm on the border with this topic. The realistic side of me says there's nothing wrong with having and owning multiple domains but I can also see the benefit you discussed when you said, “all of your efforts and all of your content and all of your links focused on one single site.”
Sure, you're always going to have one primary website that you will ultimately have the most success with and that's fine. However, what if your primary domain gets hit by the Google Bot (also known as Black Death)? What would you do then? I have a great example of one company's intention to not focus all your efforts into site.
I've been following the "loans" space for about 4 years now: car loans, mortgage loans, payday loans, cash advances, personal loans, etc. If it's a loan I've tested it.
Below is a list of websites that are all owned by the same person who is obviously using the "more is better" online strategy. If you type in the major keywords for any loan type (Examples: car loans, auto loans, personal loans, etc.), you will see that this person is successfully trying to position their sites to be anywhere on the first five pages of the results. I talked to the owner of these sites and his logic is that you don’t want to put all your eggs in one basket.
It’s not hard to realize that all these sites are owned by the same person and from the rankings I'm seeing, this person is doing very well.
