Because this is implemented by a major, very popular e-mail provider, it effectively reveals your real e-mail address to spammers, who can just look for this pattern in any address in the '@gmail.com' domain and strip away the + part. It will keep only the "honest" bulk mailers out of your inbox, not hard-core spammers.

This type of thing can work, but only for a small-time service provider whose plaintext encoding scheme is not widely known. (Security thorugh obscurity.) Even the hard-core spammers won't sift through millions of e-mail addresses to crack some plain text scheme that is used by two or three of them.

Also, you need the option to permanently destroy one of these, so that you never see mail from it again. No filtering bullshit. Google should control the exact set of anonymized addressees attached to your account. When you destroy any one of them, any further attempt to send to it should result in a non-delivery notice (SMTP bounce).