This is a usability issue.
One very important feature for this desktop environment is file type associations. That is, when the user can double click on a file and have it opened by a useful program which treats the file in a way the user appreciates. For example images are displayed in an image viewer, not a text editor.
At the moment, there is a major problem with the files storing contacts, todo lists and calendars because it is not obvious to a user what to do with them! When they are double clicked, gedit is opened as a fallback to display the files. What should happen is Evolution should be opened and then offer to import or view the data contained in those files.
There are some separate applications which are set up in this way:
-XFCE's Orage calendar will open an ics file and display it, without any hesitation. The ics file is just modified where it is instead of imported into a specific database.
-Pimlico's Contacts application will open a vcf file and offer to import contacts contained in it, one by one, with an option to just view each contact instead.
The reason why that kind of functionality is important is because needing to start in Evolution and then choose Import from there is incompatible with a file-centric shell. (A direction that GNOME may be heading in and currently supports alongside an application-centric shell). When a user wants to import his calendar and contacts from another source using ics and vcf as a transport medium, he is likely to be confused.
thank you for your bug report, that's an upstream wishlist request