http://bugs.winehq.org/show_bug.cgi?id=12464
--- Comment #18 from Dan Kegel dank@kegel.com 2012-06-01 11:10:44 CDT --- The common use case is "I've got this one windows app...", not "I want to simulate an entire windows system".
We can't rely on xdg-open being there yet because there are a lot of RHEL 5 systems out there. Just yesterday I got grief from someone with a lot of RHEL 5 deployed (and unable to upgrade to RHEL 6 for now), who was upset that wine didn't build and run properly there. So, until the installed Linux base really does uniformly have xdg-open, we should support falling back to gnome-open and kde-open. It's not hard.