Not to mention the evacuation benefits. But yeah, it might allow for a Everett/Seattle/Tacoma/Olympia metro area, stretching along the river.
But no way will people put up $20 a day or more to ride it.
We're looking forward to the Sounder adding more runs so it becomes a viable commuting option. As it stands, they only run a couple of times during commute hours...miss one train and you're stranded. Not good.