If you publish your keys, random others can link the key to the published ID (e.g., email addr). This means, at least with the PGP UI, that if Alice encrypts to Bob and Carol (who don't know each other, but Bob publishes his key and Carol downloaded it) then Carol can see that the message is encrypted to Bob. If Bob and Carol are adversarial, or if Bob is concerned about traffic analysis and message confidentiality, that's bad.
(Of course, if you privately distribute your (several) keys, you can prevent this.) Another reason the public 'web of trust' is nought but a social-network documentation device for TLAs, much like server-held address books (or even emailing lists?). The flip side of 'information wants to be free' is that sometimes its *your personal* information :-)
