xdg-mime searches only defaults.list for an application to support given mime type. The patch at freedesktop bugzilla makes it grep mimeinfo.cache too. Reproducible: Always Steps to Reproduce:
The mime info cache cannot be used the way you propose. It is a binary file and the patch you propose upstream just doesn't work as far as I can tell. In any case, if you want such feature, please work with upstream first, they will be better at guiding you to do the right thing.
Of course it can be grepped. You probably mix it with /usr/share/mime/mime.cache which is in fact a binary blob. mimeinfo.cache (used for example by Thunar) is a pure ASCII file with almost the same format as defaults.list: mime/type=list.desktop;of.desktop;files.desktop;files.desktop; OK. I'll leave this bug closed and work on it with upstream.
