Trust: there's no “DNS level” which you can reason about reliably across the wide range of networks people use. Developers would still get bug reports because some ISP resolved localhost to the IP address of their search / ad page, or a dodgy home router returned its setup page, etc. Lest that seem contrived, there are major ISPs – national level in Europe – which ran transparent HTTP proxy-caches which stored pages for years beyond their expiration date. I have zero confidence that some ISP wouldn't through incompetence or marketing have localhost resolve to something you don't expect.
Building it into the network stack means that entire class of errors stops happening.
oh, screw off. Run your own servers and mandate your configurations for search and acceptable servers or go
tsig and penalize your providers. this isn't hard. In fact
for a scalable|good business it should be mandatory.
Building it into the network stack means that entire class of errors stops happening.