The plain keyword is used to avoid having the content being inserted put inside a tag. By default, the content is inserted inside a <span> or a <div> tag (<span> is used if the output does not include <div>, <p> and other such tags.)
Although one could overload the theme_InsertNode_node() theme function, it is at times practical to be able to use both methods.
The plain keyword does not otherwise generate any output.
This keyword primary use is for adding CCK parameters inside HTML code as proposed by one of our users (see here):
<input type="hidden" name="item_name" value="[node:book_abc cck=title; plain]" />
In this HTML example, we insert the title from the specified node named book_abc and make sure that no extra tags are included.