html content signatures and web bugs

Heya.

Can anyone tell me why I *shouldn't* block inbound mail whose HTML body
contains either Javascript or embedded HTML forms?  I'm doing this now,
but I'm not sure if I'm merely asking for a chomp to the gluteus.

And while we're at it, does anyone have a reliable regexp for
identifying web bugs?  I currently use:

 
<img[^>]+src=.?https?://[^/]+/[a-z0-9./%-]*[?=][a-z0-9\./%-]+[^>]+(width|height)="?[01]"?[^>]*>

(case insensitive of course), and this catches zillions of bugs, but I'm
unsure whether I'm missing any alternate formats.

Any advice?

Tnx.  :)


-- 
  Paul Chvostek                                             
<paul(_at_)it(_dot_)ca>
  Operations / Development / Abuse / Whatever       vox: +1 416 598-0000
  it.canada                                            http://www.it.ca/

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