Jonathan and I discussed this, but I lost track of whether we concluded anything.
I think it's probably used more when people mistake it for the main search bar than for anything useful. About all it's good for is jumping to a project without going to any pages that use that project, which is mainly useful for a project that isn't used on any pages, i.e. one that doesn't exist yet.
Since a project with no files is (I think) useless, and you put files in either by importing or by typing into a page, shouldn't a link to the Import page be sufficient? Or maybe you need to create a project by attaching it to files that are already in the wiki?
Also, if we do need a way to type in an arbitrary project name, maybe it would be better to add the function to the main search form? Like, where it says, page "Frobenius" doesn't exist, click to create it, I could add a line that says, also WorkingWiki project "Frobenius" doesn't exist, click if that's what you want to create... And I would want it to return positive search results in ProjectDescription: as ManageProject links.
Anonymous
A project with no source files and no project-file tags referring to it is in fact useful, but as a sort of file-dump that other projects store things in. In which case you create it by adding it as a prerequisite. So it doesn't change this question, just thought I'd mention that for correctness.
Anyway I don't think the project search bar is useful, and it's bothering me, so I'm going to disappear it.