Well, as long as we're conjuring images of the future, I like this one: Turn on TV and see weather forecast, walk into garage and cover bike seat, don rainsuit, ride off. 

Yup, that would be a great plan if we could actually depend on weather forecasts. No matter how many computer models and tracking stations they have these days, I find I can count on an accurate forecast about as often as I could in the old days when we all assumed they just used a dart board to make their guesses.
Yes, if you actually know it's going to rain, or it already is raining before you set out on the ride, you can easily cover the seat beforehand. However, in my long time riding bikes I rarely started out under that scenario. My experience has always been more like what Twolanerider posted. Riding along, and that 20% chance of showers turns into a 100% chance of thunder storms and gully washers as I'm riding. In that situation, unless there is a convenient overhang you can quickly duck under, you and the seat will be soaking wet before you can stop to don the rain gear and cover the seat. Covering something that is already soaking wet seems to be a waste of time and effort to me.
Anyway, if you're the type who willingly starts out on a ride in the rain, then your plan sounds like a good one. Just don't be surprised when Mother Nature throws one of her curve balls at you.

Jerry