Whereas on private servers the chances for off hands and shields were always 75% essence 22% dust as with weapons, it seems to be the other way around on classic as per armor distribution.
I'm currently too poor to reliably test, however almost all shields I have disenchanted have resulted in dust. I may have gotten unlucky; my sample size is only 20-30 or so, but all other disenchants I have done were as expected so you may want to keep an eye out.

Thanks, fixed. Off hands was already correct actually (guess they were wrong on private servers).