thats not true one of my friends was racing a hartail in the fully categorie just because he wanted to race against all of his budies and a few people actually raced their hardtails on both cateogies just so that they got to have extra runs. they really couldn't care less at all so whoever told you that is bullshitting you.
i already said why the hell would you want one? it would be compeltely pointless since the chainguide drops down lower then the chainring anyways so it would be what gets hit off ricks and stuff instead.