The problem is not the transport format - HTML is no worse than JSON or XML - it's the lack of a standard schema.

For e.g. Wordpress to import Tumblr blogs they need to write an importer just for Tumblr's format, whereas if Tumblr used a standard like RSS, WP could use a generalist importer.

Don't underestimate the value of standard formats; the transport mechanism (XML/JSON/HTML) is a small detail.