I've been a pro painter for over 30 years and can also offer some opinion's. painting the underside,trunk and engine compartment with SS will not get that clearcoat haze from light scratches and scuffs. it can be more durable depending on the quality of paint. you can also wet sand any small imperfections before painting with BC/CC and you might achieve better hold out on the BC/CC job.
BUT your jambs and such won't match the BC/CC paint.
the number 1 reason I think they do it is for cost. the SS is cheaper and you need a lot less BC to achieve proper coverage.
for a nice driver it's not a bad idea to paint the trunk,engine compartment and underside with SS IF the color is close. the exterior not so much.
Sounds like they are cheaping out on the job and spinning it as a high end technique.