Some people will buy these cars to track and race down the 1/4m, get performance metrics from etc.
Not everyone buys a car to tootle to Tesco and back at 3000rpm. When you buy a performance car, you buy it for its performance otherwise you would opt for a 1.6 TDI base model version. If you're buying a car to take onto the tracks or do timed runs etc then you want that car to weigh as little as it possibly can. You also want a better handling, better braking car too. Adding 30-40kg of weight doesn't just slow the car down in its acceleration. Stopping distances are increased, handling and feel due to the higher C of G suffers, it's heavier on brakes, tyres, etc etc. There are people out there that fit lightweight wheels, brakes, batteries, seats etc to make their cars quicker so it's not hard to understand that people have these views about adding unnecessary weight to their cars to begin with
I'm sure there are S and RS owners out there that just buy the car for the badge though and don't bother to spool their turbos beyond 3500rpm so overall weight and performance is not high up in their priorities.