Or they need it to convince people like you, that the Model 3 really isn't an improved Model S for halve the price.
Of course they don't market the 3 to it's full potential, it isn't even out yet and they have way too many reservations already. But they won't stop selling the S, when the 3 is out. They will probably upgrade the battery, maybe even the motors and inverters, but that's it, the S will still be a lot better, but also a lot more expensive.
I think they need the sheet to show potential S customers, that they can't just wait halve a year, to get the same thing for halve the price, which they actually won't. The current Model S's biggest upstage will be the Model S of December 2017, not the Model 3.