[development] Block visibility

Hi, I just noticed the introduction of the option to use PHP code to control visibility (on the block configure page). Is there any documentation on this or any links to the conversation that led to this decision? Besides being IMHO decidedly un-user-friendly, this could very likely lock an admin out of his entire site. I couldn't find any information on drupal.org or in the mailing lists. Apologies if this has already been addressed elsewhere. Thanks Karthik.