First, to get the irresistible pun out of the way, I would say it is a question of semantics :)
More seriously though. To me, SMW resolves an important problem in the Semantic Web : Authoring.
there was two basic ways to generate content for the Semantic Web : 'triplify' existing content or generate Semantic Web ready content out of the box.
SMW provides a seamless way for people to author content that can be consumed using Semantic Web techniques, with the familiar interface of a wiki. Semantic Web is often confusing because the complexity of formats tends to turn people off. SMW is a good way to produce content without having to think about RDF. Compare that to editing HTML with RDFa, especially since there is an RDFa extension now :)
I am sure tools are coming to help author Semantic Web content more easily, but SMW is available right now.
I saw a presentation about semantic wikis that stated 'Semantic Wikis are the Semantic Web in a box'. In my experience, that's exactly what happened with SMW. It allows us to put in practice Semantic Web concepts with very limited integration work.
I can try to elaborate on that later - I have to run for now - but I wanted to reply to the question anyway.
From: "Yaron Koren" <email@example.com>
Sent: Wednesday, September 23, 2009 5:22 PM
To: "Semantic MediaWiki users" <firstname.lastname@example.org>
Subject: [Semediawiki-user] Is SMW a Semantic Web tool?
"Semantic MediaWiki (SMW) is a free extension of MediaWiki - the wiki-system powering Wikipedia - that helps to search, organise, tag, browse, evaluate, and share the wiki's content. While traditional wikis contain only texts which computers can neither understand nor evaluate, SMW adds semantic annotations that bring the power of the Semantic Web to the wiki."
I was looking at this, and was tempted to re-write the last part to something like, "that allow a wiki to function as a collaborative database", because I thought that gave a clearer idea of what SMW actually does; but then I thought this was part of a broader question that maybe deserved general discussion. The question is: how tied in is Semantic MediaWiki with the Semantic Web? Is SMW basically just an offshoot of the Semantic Web (as the intro has it), or is the Semantic Web, in the form of RDF export and the like, a minor, disposable part of SMW? Or is it somewhere in between? I tend to think that SMW and the Semantic Web, though both important, are basically independent concepts; though I might be in the minority on that one.
There's also the somewhat-independent question of how SMW should be marketed. Does it bolster SMW to associate it with the Semantic Web, or does that just confuse people and/or make them think SMW is basically an academic exercise? Obviously the answer depends a lot on whom you're marketing to, but what about cases like the SMW homepage, where you have to have one set of wording for everybody?