(Said this in the Wii U thread, this move is basically designed at taking Sony out of the game)
If the rumours are true about rough parity between Nintendo and Microsoft, and I can see that being the case, then it basically kneecaps Sony. Especially if they are running later, which they probably are.
As far as Microsoft is concerned, their battle was won this generation. Everything was thrown at the 360 to stop Sony gaining dominance of the living room. It worked better than they could have hoped, and reversed the previous generation. Their focus has always been services and content delivery, locking people into Live. All they have to do is reinforce it now, and they have Kinect as their other angle. They don't need to repeat the same approach as last time, their battle has been won.
Nintendo's priority is just to maintain parity with the others, so they inherit all the 3rd party games they missed out on this time. And if the rumours turn out true then they were obviously pretty clued-up on what direction Microsoft would take. Nintendo will have the tablet to define themselves, if they can solidify all that with a killer-app and get the message across simply, plus their natural strength of 1st party games.
There is no desire from publishers to sky-rocket the costs of development any more than needed, this generation has been damaging enough. The risks are getting too high. So with Nintendo and Microsoft drawing the battle line, it puts Sony in an impossible position. If they come to market later, higher-specs won't have much impact because the market has changed and games will be developed for the easiest common denominator. Plus the systems will most likely be cheaper, and have been on the market a lot longer. Especially in the case of Nintendo.
So it puts Sony in a difficult position of how to differentiate the PS4. They don't have the Wii or Kinect to play off, Move has bombed, and going the high-spec route later is a very high-risk strategy without any guarantee of reward.
It's a pincer-movement by Nintendo/Microsoft.