Justin James notes that distributing an updated version of the application is much easier compared to pushing out a new desktop application. Take this poll to let us know whether you think version numbers still matter.
One of the interesting angles of Web applications is the deployment model. "Deploy once and run everywhere" or something along those lines. As a result, distributing an updated version of the application is much easier compared to pushing out a new desktop application. In a clustered environment, the transition can be invisible to end users, even if something goes wrong.
The end result is that Web applications tend to slowly morph over time in comparison to desktop applications, which go through the giant leap known as version upgrade on a periodic basis. Over the years, desktop applications often started getting year numbers instead of version numbers (at least not in the product name, although it's usually still buried in the About window). And when applications get released every year, they often lack reasons to upgrade.
J.JaDisclosure of Justin's industry affiliations: Justin James has a working arrangement with Microsoft to write an article for MSDN Magazine.