While I agree that many parked domains are misspellings used by phishers and/or the grabbings of cybersquatters to build ad farms, there are also legitimate reasons to park a domain. One is to bag it before the cybersquatters do, pending website development. As a web developer, I routinely grab a domain for a client as soon as they approach us for a job, and park it while the site is being developed.
Sometimes the client may take a few months to get organised and actually call for us to start the job, in which case hanging onto the domain ensures we have it when we need it. It also gives us some protection if another developer tries to hijack our client, since if we do the CNA (Client Needs Analysis) for them (which costs us money) and the client then decides to go to another developer we'll add the cost of the CNA to the charge for transferring the domain name to the competitor.
But yes, there is a right way and a wrong way. We park domains by pointing them at a generic "Under Construction" parking page on our server, not on an ad farm. Not just out of civic-mindedness, I'm afraid; the reason we don't show ads on parked domains is mainly because we don't want the domain coming up in search results when people google for porn!