That makes someone else's content depend on the availability of your content - in the example of comments, the context and any intermediate replies can be lost if you stop hosting them. Leaving not a record of a conversation behind, but only indecipherable fragments.

A quick scan through the page at the link posted also seems to indicate the protocol depends a lot on content providers being well behaved. I'm assuming that somewhere in there there is handling for when content simply goes away with notification?