I agree that with todays cleaner running engines and gasoline, FSC aren't needed(as much/if at all). But, as stated in most owners manual that, there is no need for fuel system cleaners, really pertains to...the Mfg's don't want unsuspecting car owners to just buy any after market fuel system/injector cleaner and dump it in your gas tank.
Maybe the Mfg's should provide a list in the OM or, a type of FSC that is suitable for their fuel system such as mentioning PEA for example. And also state that..."if there is a noticable power deficiency, to use, X or Y brand.
But again, the Mfg's don't want owners putting in just anything so, they've made their statement to use nothing!