Thinking long term, I think that pop-ups and pop-unders are bad for the PPC parking industry.
If direct navigation becomes annoying, then people will be less likely to use it. I think parked pages should be fast and relevant to what the person typing in stuff to their address bar is looking for.
Also, there are some pop-ups/pop-unders that are scammy, like the ones that say stuff like “You have won a laptop computer! Click here, enter your e-mail address, and we will send it to you!”. If a new Internet user believes the pop ad but does not get the prize or gets spam, then that person will probably have a negative opinion of parked pages like the one that served him that ad for the rest of their entire life on the Internet. Think long-term people.
Generally, almost any website or parked page that causes pop-ups or pop-unders gets a negative vote from me. HTML tricks were interesting in 1996, but in 2006, most users will find them annoying.