Web of Trust does not allow you not to be "involved in the CA / RA process". It is quite the opposite: with a Web of Trust, everybody is involved in the CA / RA process. That's the point of WoT: any single user can act as a CA+RA, by verifying the physical identity of other users (the RA part) and then signing their public key (the CA part).
An often overlooked aspect of Webs of Trust is the need for redundancy. To understand it, consider the certificate validation process. User A (let's call him Akhenaton) has a copy of a public key which is purported to be owned by user B (Burna-Buriash). But he wants to make sure of it before using that key for encrypting some diplomatic letters. Fortunately, Burna-Buriash's key was certified by Suppiluliumas: the latter signed (with his own private key) a document containing both Burna-Buriash's key and name. Akhenaton does not know Suppiluliumas' key either, but this key was in turn certified by Tushratta, and Tushratta is Akhenaton's friend and Akhenaton knows Tushratta's key. This allows Akhenaton to verify the signature on Suppiluliumas' certificate, and thus Akhenaton gains some trust in Suppiluliumas' key. He can then proceed to verify the certificate issued by Suppiluliumas to Burna-Buriash, and thus gain some confidence in the key which was alleged to be owned by Burna-Buriash.
Now we see that in the certificate validation process, Akhenaton had to trust both Tushratta and Suppiluliumas to do their job correctly. Indeed, while Akhenaton has first-hand knowledge of Tushratta's key, he must still somehow believe that Tushratta will not try to deceive him by issuing a fake certificate, one containing Suppiluliumas' name, but a key which is, in fact, controlled by Tushratta himself. Tushratta's signature protects the process from interference from third parties (like that never-do-well scheming Aziru), but they do not provide protection against treason by the participants themselves, or even gullibility: Tushratta might have drunk a bit too much of imported Mycenian wine that day, and might have been himself deceived by Aziru, wearing a fake Hittite beard and claiming to be Suppiluliumas.
Things are worse at the next step, because while Tushratta is a personal friend of Akhenaton, Suppiluliumas is unknown to him. Akhenaton has no way to assess the trustworthiness of Suppiluliumas; even if Akhenaton supposes that Tushratta is friendly and sober and that the key which is purported to be Suppiluliumas' is indeed owned by Suppiluliumas, this says to him nothing about whether Suppiluliumas will truthfully do the same RA job with Burna-Buriash.
To state things simply: trust degrades sharply when transferred.
Hierarchical PKI (like what X.509 was designed to support) and Web of Trust PKI (the normal model for OpenPGP) deal with this issue in distinct ways:
In a hierarchical PKI, the structure is pyramidal, with few CA compared to the user base; say, one hundred CA for one million users. Each CA is bound to the upper CA (which certified it) by contractual and legal requirements, so that any mistake or fraud will imply heavy retaliation on the faulty CA. CA are thus "guaranteed" to stay in line and be trustworthy by threatening them with all the Might of the Law. This can work as long as there are not too many CA, and the whole structure is properly centralized.
In a Web of Trust, every user is a potential CA. A WoT relies on the following assumption: there may be some black sheep in the lot, but the majority of users are honest and will not try to cheat, and (possibly) most of them will be cautious enough not to sign each other key after the third pint of Guinness. There is a bit of wishful thinking in that assumption, but let's believe it for a moment. In WoT, you cannot gain enough confidence in a key by verifying a single certificate path; instead, you must verify many paths going through distinct intermediates. This ensures safety to some extent because while some paths may go through cheaters, not all users are corrupt. That's the redundancy I was talking about.
Since redundancy is important for WoT, then user software involved in WoT-based validation MUST support it. The software must build several paths, check that they do not intersect, and verify them all. That's where S/MIME fails at doing WoT: although nothing prevents X.509 certificates from implementing a WoT model, X.509 was not designed for that, and, correspondingly, software which does S/MIME (i.e. email applications like Outlook or Thunderbird) just assume that they are in a hierarchical PKI model and declare themselves to be happy with a single verified path.
That's the important lesson here: the PKI model you choose must be implemented by the verifiers, i.e. the users' applications. If you want a Web of Trust, users must use applications which actively support it, and, currently, this means OpenPGP format (with, e.g., GnuPG).
In the end, Akhenaton trusted everybody. This did not turn well for the Egyptian empire.