You don't need DVCS for more than one developer, but certainly for some teams, organizations, and situations it offers a lot of benefits.
Fir a lot of projects the DVCS advantages are tenuous. A lot of regulated industries mandate by law fairly strict top-down control that erodes a lot of the DVCS benefits. A lot of projects are not pure software projects that have lots of required binaries files that git and mercurial are powerless to merge sensibly. A lot of organizations have a variety of projects all stored in a single repository for whatever reason, and having to clone the entire thing for a single project is a nuisance.
For situations like those, which are commonplace in the commercial world, SVN and the centralized model is a fine way of doing things.
In my experience, DVCS is excellent for pure software projects, and sucks for anything else. SVN on the other hand is a great tool that is useful for a great number of projects beyond software.
In otherwords, use the tool with the right capabilities for the task at hand, not because of what's trendy.