Nobody is springing to the defense of this so I'll only say that it's a time-honored practice and we shouldn't be so quick to disparage it, lest we be disparaged several years hence for the things we do :-)
It's rotten, and when I typeset books (http://www.evertype.com/books.html) I always have to clean up the text which is invariably littered with these artifacts of old technology.
In the world of plain text, two spaces after a sentence-ending period, exclamation mark, question mark, or other mark is actually rather handy to distinguish sentence enders from the same marks used in other ways, esp. periods in abbreviations.
Fie! Fie! Unclean! Unclean! -- Michael Everson * * Everson Typography * * http://www.evertype.com
