Version 24.3 of Emacs seems to have eliminated some functions/variables that pure-mode depended on to work properly. If I run pure-mode and try to insert an open parenthesis, I get an error:
Symbol's value as variable is void: last-command-char
and nothing happens (i.e. no open parenthesis is inserted).
This is apparently becuase this variable is "obsolete" and was removed, according to the news file accompanying Emacs.