The real mystery to me is why Craigslist shut the service down, because (protocols aside) it wasn't fundamentally different from any other way of consuming craigslist. Craigslist's terms of service is actually pretty confusing:
USE. You agree not to use or provide software (except for general purpose web browsers and email clients, or software expressly licensed by us) or services that interact or interoperate with CL, e.g. for downloading, uploading, posting, flagging, emailing, search, or mobile use. Robots, spiders, scripts, scrapers, crawlers, etc. are prohibited, as are misleading, unsolicited, unlawful, and/or spam postings/email. You agree not to collect users’ personal and/or contact information (“PI”).
...which if you think about it, bans the use of RSS readers too. There's a short blurb on the RSS page:
craigslist RSS feeds are for your personal use only, and are not available for commercial use without first obtaining a license from craigslist. Please consult our <Terms of Use> for more information on using craigslist RSS feeds.
The only relevant part of the Terms of Use is the section mentioned above, which doesn't really shed any light on the situation.
Honestly this is really confusing. Once again, Craigslist disappoints.