I do prefer thin WHOIS, because every party has to provide the information which can be verified by the party itself and for which the party is liable directly (they simply has to know their contrators/customers/resellers).
All thick WHOIS includes the identity of the registrar, so this argument is silly unless you think that registries are so incompetent as to be unable to publish the information that the registrars provide via EPP. (If you do think that, examples would be helpful.) The only honest reason to favor thin registries is as a backdoor way to hide the identities of registrants who are criminals and the far smaller set of registrants who (probably wrongly) believe that A) their personal contact information is secret and B) they would suffer significant harm were anyone other than the registrars, who are perfectly reliable, to know who they are. Having written more than my share of software that attempts to retrieve the WHOIS info of people who submit their contact details to abuse.net, I can report from experience that the current thin WHOIS does indeed often fail to provide even the information that the registrars have. Thick WHOIS, on the other hand, is reliable and consitent. I also note that this particular question says nothing about proxy WHOIS, which is available in thick registries to exactly the same degree it is in thin ones. R's, John