We're not talking about centrally-developed distributed applications; we're talking about services that tell you to talk to other services. Services that could very well be developed by other people/organizations, years earlier or later. In fact, services that might very well be "plugins", in the sense that those URLs are user-supplied content from the perspective of the API. (Think: permalink URLs from user-supplied RSS feeds, in the responses of an RSS feed-reader API. Or image URLs, hotlinked in an oldschool phpBB forum. Or RDF vCards.)
Seems like a premature optimization to me. Building all applications so that they potentially could be distributed over multiple servers. YAGNI.