They certainly are worth parking. After all 5 lousy visits per month and 1 miserable click can be enough for a domain to pay for itself.
All depends on volume, industry & optimisation IMO
If you are selling the domain (or would like to consider offers), make sure you note this on your landing page. Potential purchasers (esp end-users) often navigate directly to the page and a simple "This Domain May Be For Sale" makes all the difference in getting offers.
If you park with Namedrive, they'll automatically provide a link to your listing on Netfleet too.
I think Sedo also provide the same service pointing towards their sales listing.
But definitely park your domains - anything is better than an unresolved page (even for the casual web visitor)