Sir you are absolutely right that there is no need to put undercoat of a new car but in case of suzuki, they use w=very thin layers of paint beneath the car. I have 2016 mehran and despite regular cleaning, the wheel arches have started to rust just because the paint coat.
Day before Yesterday we took delivery of alto VXR. believe me it was like this coat in the wheel arches. he suzuki ppl are evil, they can do anything to reduce cost. even the car standing on showroom display had rusty nut inside the wheel arches....i wonder how they would be beneath the car. so if u have budget and to avoid hassle, get it done simply.
As for showrooms asking own, i would suggest to simply take secret camera, make a video and post on social media. this problem will get resolved automatically