well, a few things
masturbatory v. expressive is a false dichotomy. when exactly does expression become masturbatory? is there some specific level of technicality, prolificity, "flash"?
you also seem to be making a bunch of weird value judgments about purpose, utility, artifice
like, why is flash bad? why is it worse than "developing the melody"? would you make a similar argument about blocky rave-stabs and uplifting pianos in ardkore?
and how is the Edge's (or Kevin Shields's) pedal set-up not masturbatory? like, a lot of notes is bad but a ton of layers/fx is good? really, neither is good or bad, just to point out it's a silly argument.
it's true plenty of King-style atonal shredding could easily be transposed between songs, but you could say that for popular motifs in virtually any music. how much drum choppage in jungle, how much post-punk angular guitar skronk, is basically interchangeable? is it a point against them?
every genre has sonic markers - shredding is indistinguishably part of the current running from the harder end of thrash metal (Slayer, Dark Angel) thru proto-death/thrash (Possessed, early Death) thru classic death metal ca. 1987-1992 or so (after which death metal splits in a bunch of directions). go listen to Altars of Madness, Morbid Angel's 1st LP and probably the quintessential death metal qua death metal record, and tell me the shredding isn't intrinsic to the music. not just the notes but the way they're played, and the techniques - dive bombing, pinch harmonics, etc - used. you'll also notice that the lead/fill/solo line is blurred - there are shredded scalar fills all over the place.