I would be happier if the active components of Theatre View, such as menus, program descriptions, metadata, episode images, etc. faded completely to transparent, and the Theatre View slideshow was left playing on the screen, rather than having the screen go black. That way the screen is protected, I can see that it is on, and I can see the slideshow images properly.
Maybe completely black is one option, and just slideshow is another, or probably even better, just allow variable opacity of active components from 0 to 100% after a wait period. I believe this idea has been discussed before.
On why the screensaver isn't working:
Any active connection to the server, including DLNA clients somewhere on the network, whether defined in MC or not?
Any background process in MC, such as Thumbnail creation or audio analysis? Hard to check when waiting for a screen saver to kick in, but easy to check if all thumbnails and analysis is complete: "Help/System Info" or "Services & Plug-ins/Reporter". Check the "Background Tools Running" item as well.
If you put MC into Standard View and minimise it, does the screensaver work then? i.e. When MC isn't visible on screen.
BTW, screen burn-in was a serious problem with Plasma screens, but it really isn't with a good quality LCD/LED screen. Do you have a plasma?
PS: I just tried turning on the Windows screensaver on my HTPC, just to see if it would work. Yep, worked first time, every time. First with the Bubbles screensaver, then with a slideshow of images. I am using RO Std, no background activity, Hardware Acceleration and Video Clock on, Display Settings automatic change off, no cropping of video, no Bitstreaming, Windows Size at 100%, Windows Text Size at 125%, Theatre View size at 100%. Basically, pretty much default settings.