Another reason for pd is the license and that it runs on a free os. This is interesting when you do art installations and dont want to buy a os license for each box like windows (yuck) or worse, buy a propritary box and os license like Mac OSX. Plus you don't need to fiddle around with dongles or response code registry stuff to install it. This can be important when your patch misbehaves on location and you need to edit it although your available computers already had ran through the whole Max demo period. PD feels better in such situations...
Cheers,
Malte Steiner media art + development -www.block4.com-