What can I expect when a certain template maker affirms that his template supports K2 or places a K2 component in his showcase template? Or displays his template in the 'extend K2' section of this site?

What I expect(ed) is that I can use K2 content instead of Joomla content. No Joomla articles and categories, only K2 items and categories. And obviously that all extensions delivered with the template support K2 content too.

But this is not the reality. My last frustrating experience was with a JoomlArt template called NEX which can also be found in the 'extend K2' section of getk2.org. After many hours of installation and configuration I discovered that the sidenews module does not support K2 content - and there seem to be other restrictions. So what is such a "K2 ready" template good for?

Does it make sense using mixed content? Some Joomla articles and categories here, some K2 items and categories there? Is the 'K2 ready" template a "Fata Morgana"? Or am I member of the wrong "club"?

Are you a member of the wrong club? Not necessarly. May be that you can substiture another extension that does display k2 for the one that doesnt.

does it make sence to use mixed content? quoted from Jen4web " it depends"
Many people do that, use articles for what they are best for, and k2 items for what they are best for.

If your K2 ready template has a built in category, item, and user k2 override built into it and styled to match the site i would say its k2 ready and much of the work to customize has been done for you

If you have specific questions about a specific extension please ask, and remember all the above is IMHO