These sorts of questions didn't start with Google. They're known as Fermi Problems for a reason: they're named after Enrico Fermi, the physicist.

http://en.wikipedia.org/wiki/Fermi_problem

Knowing how to quickly estimate something is useful.

I imagine that Larry Page does a few quick estimates every day. How many Loon balloons would it take to bring Internet to 90% of Africa?

But not everybody at Google has a job like Larry Page. It's gotten to be a big company full of accountants, HR people, and other jobs that don't require much thinking in unfamiliar territory.

In other words, guesstimation is a useful skill, but not for every Google employee, so it's not going to show up as useful on average.