They're no more valuable (or less) than a nonsense word because the same effort and resources need to be invested in branding to build its value.
A domain name's value is found in the relative ease by which it may be branded. Is 'SearchEngine.com' easier to brand than 'Google.com', for instance? To figure out a domain name's value you need to audit those properties that will help the name take on a unique and identifiable brand.
Even if you have an easy to recognize generic name it would still need to be branded for an end user's particular use and that still takes effort and resources.