[wp-hackers] Why kses filtered html strips class?

[Chris]

Hi,
I have a modified plugin which allows my "Editor" users to upload and 
add links to pdf files from within tinymce editor - (it inserts an icon 
with a link to the actual pdf file). I have put a class="thumb_icon" 
into the html, so that the link icons can be styled - but kses is 
stripping code out - why? and how to modify so that it doesn't.
I don't really want to allow unfiltered html for the "Editor" capabilities.
The kses.php mentions using my-hacks.php... if this is the way to go, 
what would I put in there?
But I still don't understand why class="xx" is considered nasty and must 
be removed!
thanks
Chris

-- 

    ********************************************************************** 
    Chris Hearn 
    *Tel:* 01625 827589 
    *Mob:* 07768 725141 
    *email:* chris.hearn01 at ntlworld.com 
    ********************************************************************** 
    The information transmitted is intended only for the person or entity 
    to which it is addressed and may contain confidential and/or privileged 
    material. Any review, retransmission, dissemination or other use of, 
    or taking of any action in reliance upon, this information by persons 
    or entities other than the intended recipient is prohibited. 
    If you received this in error, please contact the sender and delete the 
    material from any computer. 
    **********************************************************************