Flight will work in standalone mode with a computer disconnected from the internet according to everything I've seen from Microsoft.
I have used one other game which is like Flight - transitioned from a technical and complicated stand alone game with a multi-player interface to a simplier Windows Live game. In general, I only used Windows Live to play online. A much better on-line experience than any version of FS Multiplayer.
But (1) it was difficult to get away from the shoot-em-up kids - it could be done but took some work to figure out how, and (2) I could not find enough old farts with slow reaction times to play with, making it a not very enjoyable experience for me.
It did 'nothing' to my computer, other than a graphics heavy game normal FPS type issues.
However, we really don't know exactly how and what Flight will be. But, I'm not worried about the Windows Live on-line multiplayer aspect.
If anything, the dumbed down version of Flight will make it easier for my young grandchildren to learn the basics of flying and help them transition to the steep learning curve of FS2004/FSX.
@ PawPaw's house - near KADS, Addison, Texas, USA