I haven't seen a QuarkXpress file in 8-10 years. When I do see a QuarkXpress file, I simply use the Q2ID plug in and convert it to Indesign. But that's my office. QuarkXpress is very much still in use for large production houses where they've been using it for 15+ years.
Essentially if a business, or designer, started before the development of Adobe Indesign (Circa 1999) then they may still prefer QuarkXpress. This may be primarily to support older files or because workflows are dedicated to QuarkXpress features. And there are some places which simply don't like Indesign. Many use Indesign because it's bundled with Photoshop, Illustrator, etc and it just makes sense to stick with Adobe rather than pay the additional $1k US for a separate layout application.
While my office has little or no interaction with anyone using QuarkXpress today, QuarkXpress is still very valid for many positions.
Also, realize most help wanted ads are placed by the Human Resources department who know nothing of the software or requirements. They may have a standard ad they place for a position which includes QuarkXpress because it was needed 15 years ago even if it's not needed today. If I were looking for employment, I wouldn't let the mention of QuarkXpress or Indesign sway me away from a position. If you know one well you can transition to the other. It may take a bit more study, but if your'e worth hiring, it's doubtful that knowing Indesign over QuarkXpress (or vice versa) would prevent you being hired.