While I agree with Jensenman, I think you do need a sporty car or two to rub off on the plain ones. Toyota did a great job of it in the '80's when they boomed in sales with the MR2, Celica, Corolla variants and Supra, and again in the '90's with the MR2 Turbo and Supra Turbo. While they may not make much money, they keep them in front of the public in press and appearance. They missed out this past decade with the ugly and space challenged Spider and in my opinion the Scion branding.
Sports models also keep their engineers challenged and interested.
Remember, Nissan tried the same thing in the mid '90's and it really hurt them as a company. A good sports program injects into the entire company, look at their products now compared to Toyota.
Eventually Toyota will be hurt by this approach. Even the non-car people I know like to think their toaster has something special, such as a friend with a Malibu claiming it has Corvette DNA or something like that, even though it doesn't.
Don't underestimate the damage a non-sporting image can do to a car company. There is a reason why most every car company in the world strives to have an image like that, and why they spend the money they do.