you can also do the same sort of thing with a switch for the neutral, too, for effectively no change in complexity. . . just add another contactor. Safer, not much more expensive.
I personally would rather see the genset completely isolated from the grid. (I'm guessing so would your utility linemen)
Others have pointed out the kiss factor of making a 'patch cord' for your critical loads, and having to physically move it from grid to gen and back. (so much for automatic)
also, seems like you'd have to wander out to the genset to get it started up, so incremetal effort to operate the plug or manual switch seems trivial. Am I missing something? (autostart genset, mayhaps?)
<g>
but!
if you're going to use a common neutral, be sure and tie the generator to a good earth ground! (the gen neutral, too. . .) that way you don't end up doing wierd things to the system on switch-over or maybe wind up with the neutral to the rest of the world winding up with some potentially lethal offset from ground. (zap.. . OUCH!)
