So Paul asked recently: Does Linked Data need RDF? If you drink a certain sort of coffee, I guess you are familiar with my answer: What else? 😉
Seriously. Let’s step back for a second and try to work through to the core of the issue from a totally different angle.
Compare a set of predefined, fixed terms for certain domains, easy to use, etc. with a flexible and generic (hence, maybe, a bit more initial effort required) approach for annotating data, that is structured data on the Web. Sounds familiar? You’re right. I assume that you are aware of the old discussion around microformats vs RDFa, right? So, there we go …
Now, if one looks closer into the HTML 4 spec, one finds a bunch of link types, such as next, help, section, etc.; I’m gonna pick two, IMO, important sentences from there:
User agents, search engines, etc. may interpret these link types in a variety of ways. For example, user agents may provide access to linked documents through a navigation bar.
Ah, so the targeted consumer of the link is indeed a machine, not a human in the first place. Further:
Authors may wish to define additional link types not described in this specification. If they do so, they should use a profile to cite the conventions used to define the link types.
Ok, so there is a sort of extensibility mechanism defined in the HTML 4 spec as well. Very well! Or?
An analogy might help now to understand the point I’m trying to drive home, here. If you think back to microformats vs. RDFa, the same can be said about HTML 4 link types vs. RDF(a) …
HTML 4 link types as of section 6.12 of the spec are essentially the poor man’s semantic links, directly available in HTML. They are targeting machines (not human users in the first place), but are predefined in a sense and quite limited.
If you agree up to here by and large, then the question is really: what is the alternative? What technology out there, deployed, with community support, a set of tools available, etc. is available to represent, in a generic way (needed to write generic parser), any sort of typed link between two entities on the Web?
RDF.
What else? 😉
Note: credits go out to Juergen Umbrich with whom I discussed that issue yesterday evening and who inspired me writing the post …
Web of Data 
I have no problem whatsoever with non-RDF forms of data in “the data Web”. This is natural, normal and healthy. Stastical information, geographic information, data-annotated SVG images, audio samples, JSON feeds, Atom, whatever.
We don’t need all this to be in RDF. Often it’ll be nice to have extracts and summaries in RDF, and we can get that via GRDDL or other methods. And we’ll also have metadata about that data, again in RDF; using SKOS for indicating subject areas, FOAF++ for provenance, etc.
The non-RDF bits of the data Web are – roughly – going to be the leaves on the tree. The bit that links it all together will be, as you say, the typed links, loose structuring and so on that come with RDF. This is also roughly analagous to the HTML Web: you find JPEGs, WAVs, flash files and so on linked in from the HTML Web, but the thing that hangs it all together isn’t flash or audio files, it’s the linky extensible format: HTML. For data, we’ll see more RDF than HTML (or RDFa bridging the two). But we needn’t panic if people put non-RDF data up online…. it’s still better than nothing. And as the LOD scene has shown, it can often easily be processed and republished by others. People worry too much! 🙂
Posted by danbri | 2009-07-20, 12:06@danbri, yes I agree with what you’re saying – what a brilliant analogy (re ‘what ties together the HTML Web’), btw. Just to enforce and highlight this:
“RDF is one of the three *backbones* of the linked data cloud, equally to URIs and HTTP, tying together the Web of Data, while leaving room for other structured data, which can be incorporated through technologies such as GRDDL, RDB2RDF, etc.”
(@TimBL, can this sentence please go into the LD design issue note? 😉
Posted by woddiscovery | 2009-07-20, 12:34Hey, what about N3? Or is it to close to RDF? Anyway, it is easier to understand. I prefer N3 where I can use it.
Cheers zazi
Posted by zazi | 2009-07-21, 13:02@zazi, N3 is one (not yet standardised) of the RDF *serialisations*, see [1] for the latest state. We’re talking about the S-P-O model with URIs, here.
Cheers,
Michael
[1] http://www.w3.org/TeamSubmission/n3/
Posted by woddiscovery | 2009-07-21, 13:15Yes, I know N3 is a kind of different representation of RDF, but the full language definition of N3 can express more issues than RDF, as far as I know. So Tim Berners-Lee creates a very good language, which hopefully will be standardised in the future.
Cheers zazi
Posted by zazi | 2009-07-21, 14:14