Not quite sure that's a "hmmmm...."
Aside from the already mentioned reasons (Bad faith, cheap clicks, etc.), it also does a lot to cheapen the feel of a domain name. I know that I still have a hard time going to now-developed sites with strong keywords that were once parked. I use to check my name and others in .com , but they were registered by marchek's pred who had the most annoying damn pop-ups ever. Now, a few are developed, but I still don't like visiting because of the stigma attached to the sites in my mind...
A very touchy-feely reason, but one none-the-less.