Yeah, sometimes one can get the impression that companies build 1.4 ton cars, then pour in 300 kg of carbon fiber into them with a cement mixer and call that “lightweight”…
Anyway, I think the appeal of a lightweight slider in the engineering tab would be the ability to design lightweight exotic cars. Lowering body/fixtures quality works quite well for Delivery/Utility type demos where prestige is not a concern, but not so much for, say, Light Sports etc. Actually, maybe a prestige rather than lightweight slider (optimizing panel gaps, wind noise etc.) would do the trick so you could drop body/fixture quality with relative impunity as long as you throw enough money at it.
Another possible approach would of course be to simply reduce the prestige weighting on sports oriented exotic demos (light sports/sports), since buyers of this type of cars often don’t care much about panel gaps and stuff.
EDIT: With the latter in mind, it just might not be worthwhile to add a slider if a “lightweight engineering” effect is already kind of in the game and might be extended to more demographics with minor balance tweaks.