Lots of things I do (adding a printer, setting up a network, adding a new icon to the "dock" or "panel", setting up screensaver preferences, etc...) are things I will do once a year or two. Everytime I do them, it is like using it for the first time. Unless you are a sysadmin, you are not an expert in adding a printer. You do it once, then it's added, and you never ever need to go there again, unless you buy a new printer or new computer.
So first time use is precisely what needs to be tested for a large part of the desktop.