Pokemon missing access to certain STAB attacks can still be interesting and viable if they are compensated with other positive qualities. From a competitive standpoint, I think that requiring nearly all Pokemon receive both a reliable strong STAB and a risky-but-strong STAB would close a lot of design space. Kartana lacking access to a risky-but-strong attack like Wood Hammer or Power Whip is necessary because it's compensated by its Attack stat. Shadow Ball is a decent STAB despite being on the lower end of power for the "standard STAB" due to having relatively uncommon resists, and a widely distributed Ghost-type attack comparable to Fire Blast or Overheat would give a lot of high-end Ghosts too much OHKO power. Encouraging a Pokemon to use High Jump Kick often requires denying access to Close Combat, showing how mandating a near-universal safe(r) option (which CC is to Fighting-types in Gen 8+) can cause interesting moves which are less common to be overshadowed (while CC is in the strong-but-risky category, it is still less risky than HJK).
It's even possible to create interesting results when denying both the "standard" reliable and strong-but-risky options. Leftovers + Fly Dragonite (and similarly Leftovers + Bounce Gyarados) in Gen 6 is an interesting set that might have been overshadowed if Dragonite had access to a better Flying-type attack (and if that attack was Brave Bird or otherwise comparable in power to other "risky" moves, it might have been in outright overpowered territory). Such cases can flop massively if not designed carefully, but l'd rather have the option open to create these interesting successes over just preventing the worst possible movepool design errors. Perhaps it's fair to not trust Gamefreak to provide proper compensation for notable movepool absences, but it's far from impossible.
