I don't know. Whenever I use my friends Windows computers--and, reasonably, Windows
is the default--I'm lost. And yet this is not a good case for switching to Windows!
Besides, these days, I work from my laptop. 99% of the time, I'm on my computer. Now, perhaps I'm remoting into some server, but if so I'm doing that from a local Emacs. When I need to use a new computer, assuming it's running Linux, I only need to copy a couple of configuration files over and it works exactly how I want.
So the only time I have a problem is if I'm using somebody else's computer for a short time. This does not happen often, and when it does it doesn't last long. Moreover, as I said above, chances are high that this computer is either running Windows or OS X, so I'd be at a disadvantage even if I didn't tinker with my setup.
Finally, fundamentally, I think configuring your system is worth doing if it makes you more productive. Most of the things I've configured in Emacs do make me more productive (as far as I can tell). Things like adding new keyboard commands for operations I often use and making Emacs automatically recognize when I'm doing work stuff as opposed to non-work stuff just make life easier for me.
Configuring any individual feature is an O(1) action. Assuming it brings a benefit, this benefit is going to be in O(n) to how much I use it. Given I use both my OS and my text editor a whole lot even a significant amount of time spent configuring everything to work well pays off.
I'm not wasting my time--I'm increasing my efficiency.