And so they move over to the Mac and proceed to name files without extension names. Of course some programs like Microsoft Office and the Adobe suite have been written to include file extension names by default, but there are just as many that still adhere to the old Mac way of naming files--without extension names. And so then they go to open their files on their PC (from their flash drive, or maybe they emailed them to themselves), and Windows goes "huh, what's this?" after double-clicking. They're left thinking the file is damaged or that the Mac messed it up somehow, when the problem is simply that it's not named well enough for Windows to understand what it is.
To further compound the situation, Apple has determined that the default way of viewing files in the Finder is to have the file extensions hidden. And so sometimes the file is actually named properly but when the user looks at the file in the Finder, it doesn't show the extension name! And then they get confused and try to rename the file and will sometimes get a warning box (a scary thing for lots of new users) asking them if they're sure they want to change the file name extension.
My point is, it's really a mess. As much as I dislike having to adhere to the stupid three-letter-extension name format imposed by the Windows majority, I've decided that it's just easier to teach people to always name their files with extension names. It won't hurt the Mac to have them, and Windows requires them, so it's the least confusing solution.
I also set all the Macs I control to have the Finder always show file extensions in its preferences. Trivia: this attribute is actually stored in an invisible file called .GlobalPreferences.plist in each user's /Library/Preferences folder.