Ranking the mobile version of site A, against the desktop version of site B can be unfair in some cases.
If that was the most Unfair Thing Google had ever done I could probably live with it.
However, "Mobile Version" and "Desktop Version" seems to be somewhat polar, with some mobiles being almost tablets, some tablets having higher resolution than older desktops, and some modern desktop screens being Absolutely Massive.
Couple this with the fact that some activities (like reading research papers or sourcing B-to-B suppliers) are much more likely to happen in a desktop environment while others (like getting directions to a restaurant when you are on foot in town) are only ever going to happen on a phone, and the rationale becomes less obvious.
Even now the majority using mobiles isn't overwhelming - there are still billions of desktop users - so I'm not personally convinced it has as much to do with serving users as it has to do with driving Everyone - forcing our help! - into the market where Android is King. Things May Change, but how many Android-Desktop users do you know?
