[churchx]

It's not as lighter drive shafts don't make sense at all though, as due their sheer size/length there they still weight noticeably .. just that lightening them serve more like lightening car/it's sprung/unrotational weights in general, not so much vs lightening rotational weight of other parts that spin much faster and/or that have weights located further from rotation center, and for general car lightening there are better ways of better returns per buck or lightening by much larger weight if within limited budget, then buying expensive shaft. Interior ripping, removal of misc comfort items like AC/audio/noise insulation, lighter battery, lighter seats, rear seat removal, lighter exhaust, removal of crap in trunk, lighter wheels & brakes, lighter flywheel, removal of washing liquid tank and so on ..
If budget is unlimited and there are no class limits on that, shaft can also be one of many things to be installed for lighter weight too. But otherwise main purpose why someone should look at aftermarket driveshafts is their ability to transfer more torque of more powerful forced induction engines without breaking, lesser weight being just slight bonus of little importance, insufficient to justify the price if lightening being sole reason. Due a shape/small diameter that lightening being for rotational mass also doesn't make much difference in this particular case.