Well, I'm not really surprised or disappointed. It's something I always thought SHOULD be in the game, as long as it doesn't negatively affect single player experience.
As long as they still add all the stuff for single player that they would have anyway - I don't mind longer development, or multiplayer being developed first and single player features afterwards. I have all the time in the world.
It does offer certain unique possibilities, though.
It would be great to be able to play with some friends in the same game, and a spectator mode would be absolute gold for live streaming for example.
Also, multiplayer makes it possible to do things like... say, keep a camera crew (another user) on ground at Kerbal Space Center, recording a launch to make even more awesome films. It will be possible to do fly-by camera angles, with chase planes.
As far as I understand, the multiplayer mod already makes these things possible, but I haven't had time to play KSP, much less make videos of it recently...